Study annotated masters' games collection. The best is 1000 Best of the Best by Chess Informant. At the beginning, it's hard to understand. Later you will find out, why it's the best of the best.
What should I study next?

"I have read Silman's "How to Reassess Your Chess," Nimzowitsch's "My System," and am finishing up Watson's "Secrets of Modern Chess Strategy.""
And you're 1500 ? This is absolutely ridiculous. You didn't work these books at all, you probably just red it like as fast as some comic books. If you had really work all these books, why haven't you reach master level yet ? Do you think "My system" is for 1200 players who want to be 1500 ? Very ridiculous. And nobody seems to understand this here (even the IM, I would be curious to know how many books did this IM really red before he was 1500).

^ Because those books only address positional play. To be a master, one has to be good at all aspects of the game. So, it is very well possible that my positional play is around 1800-2000 ELO but with 1500-1600 in tactics and I don't even know how low in endgames, any positional advantage I might have could well be dissipated through tactical blows and endgame blunders.
Also, in terms of "work"ing those books, what is written in those books is written in those books. Most I can do is to read the books over and over again. But, even then, I only learn what is actually in those books. For instance, Nimzowitsch teaches much on attacking base pawns and prophylaxis. He doesn't cover much, if anything at all, on sacrificial play or opening repertoires. So, even if I master Nimzowitsch, without reading any other books, I would still barely know anything about sacrificial play and opening repertoires.

I think the Aagaard book's on positional play really helped me improve quite a bit. He talks about piece placement, shows GM's who rarely get into poor positions because they always do their best to place their pieces on good squares. I remember one game in particular that sticks in my memory, where kasparov plays bf2 and it was just a great move in the opening phase of the game. Also I remember Yermolinsky placing a bishop on a1 and I think the queen ends up on like b2, and Aagaard goes on to show how some GMs get themselves into difficult positions.
I highly recommend Aagaards books, all of them! Even the defense manual, if you have the stomache for such things.
Yeah, I heard lots of nice things about Aagaard's books. However, I heard that they are more suitable for advanced players (Class A, at the very least). True?

^ Because those books only address positional play. To be a master, one has to be good at all aspects of the game. So, it is very well possible that my positional play is around 1800-2000 ELO but with 1500-1600 in tactics and I don't even know how low in endgames, any positional advantage I might have could well be dissipated through tactical blows and endgame blunders.
Well, then my advice would be again : play some OTB long games (1800/2000 players are the best opponents for you to improve, 1200/1400 opponents will teach you almost nothing), this will clearly make you reach quickly (relatively quickly, don't except to reach 2000 after just 5 long games... yesterday I looked upon Carlsen's rating curve, he himself had to play 300 long games to go from 2000 to 2500) 1800 or even 2000. Or try to work on some endgame/tactical position, like this simple one :

If Nimzovich's system does not help you improve, Seek Tal's system. My Life and Games by Mikhail Tal is a very beautiful classic book of tactical and attacking chess.

^ Because those books only address positional play. To be a master, one has to be good at all aspects of the game. So, it is very well possible that my positional play is around 1800-2000 ELO but with 1500-1600 in tactics and I don't even know how low in endgames, any positional advantage I might have could well be dissipated through tactical blows and endgame blunders.
Well, then my advice would be again : play some OTB long games (1800/2000 players are the best opponents for you to improve, 1200/1400 opponents will teach you almost nothing), this will clearly make you reach quickly (relatively quickly, don't except to reach 2000 after just 5 long games... yesterday I looked upon Carlsen's rating curve, he himself had to play 300 long games to go from 2000 to 2500) 1800 or even 2000. Or try to work on some endgame/tactical position, like this simple one :
Thanks. I understood the first part of it but couldn't mate/promote. I drew stockfish about 20 times before it clicked :P

^ Because those books only address positional play. To be a master, one has to be good at all aspects of the game. So, it is very well possible that my positional play is around 1800-2000 ELO but with 1500-1600 in tactics and I don't even know how low in endgames, any positional advantage I might have could well be dissipated through tactical blows and endgame blunders.
Well, then my advice would be again : play some OTB long games (1800/2000 players are the best opponents for you to improve, 1200/1400 opponents will teach you almost nothing), this will clearly make you reach quickly (relatively quickly, don't except to reach 2000 after just 5 long games... yesterday I looked upon Carlsen's rating curve, he himself had to play 300 long games to go from 2000 to 2500) 1800 or even 2000. Or try to work on some endgame/tactical position, like this simple one :
great puzzle, I was reading some endgame stuff ealier today about the ideas present and was glad to apply them so quickly. Endgame visualisation is key to getting to the next level

Also, in terms of "work"ing those books, what is written in those books is written in those books. Most I can do is to read the books over and over again.
Till you get this idea out of your head, you are going nowhere.
How do you learn something that is not even written there? As I explained, Nimzowitsch's "My System" has very little on opening repertoires or endgame play (there is a chapter on endgames, but nothing on more technical aspects of endgame play, like the Lucena position, etc.). I might be able to glean some opening/tactical/endgame knowledge from Nimzo's annotated games, but even then his annotations focus on positional play and the openings he plays are not as frequently played anymore (e.g. French Defense - I don't play it and few of my opponents play it against me).
Besides, I do work on what I read. I take careful notes, play games using what I learned, and annotate my games afterwards.
re: my rating here - If you look, I only play Bullet and (some) Blitz games here. In Bullet games, I barely have enough time to think, let alone do deep concrete analysis of positions. My last standard game here was about a year ago. So, I am not kidding when I say I don't play serious games here.

Anyways, my original question (which is still my question, though your helpful replies have helped answer it for the most part) is:
What should I study next?
I am not asking for which books to buy or which GMs to follow, though advice on which books/videos/etc. I should look into is helpful and much appreciated. I am simply asking, based on my rating, etc. whether I should study openings, tactics, endgames, or continue to study positional play next.

Anyone have any other thoughts? I will be looking over your suggestions carefully over the next few days to determine what I should study next. Your recommendations and advice are much appreciated.

Analyse your own games, find mistakes and count them. If your most common mistakes is in endgames then study endgame and so on!

Analyse your own games, find mistakes and count them. If your most common mistakes is in endgames then study endgame and so on!
If your most common mistake is in the End Game then you have serios problems. Need stop playing immediately and practice before playing here.
Study where you first go wrong in games. I say pawns are 50% of the reason why games are lost in some form or another.

But my point is that if you analyse your own games, you know where all your mistakes are and you can work with them.

But my point is that if you analyse your own games, you know where all your mistakes are and you can work with them.
Think about it ..... if you know where all your mistakes then how are you making them in a game. In many cases you have to know what the mistake is ..... knowing you made a mistake and what is 2 different things.
Take for example your oppoent won a piece from you by force by means of a tactic "Overworked Piece". One can avoid this by not allowing it when they make a move that creates it or your doomed to keep repeating it. If one is not famaliar with the term chances it will happen over and over and over again.

This is usually what I do:
1. Play game
2. Run game through quick compute analysis to quickly see if I made any tactical blunders. If I did, I will read the computer's recommended line and try to play it over in my head.
3. If there weren't any obvious tactical blunders, then i look through the game carefully to see if there was a point where the momentum shifted from me to my opponent (e.g. in a recent game, neither side made a tactical blunder, but I made a positional blunder that allowed my opponent to obtain and use a Knight outpost).
4. I then determine what I messed up on and how I could fix it. If I do not know what the mistake was, then I suppose I could post the game on this forum or another chess forum and go over it with more experienced player. However, most of the time, I know what the error was.
Re: if you know where all your mistakes then how are you making them in a game.
The issue is that sometimes, I forget what I learned. So, I make the error. When I review the game, I then remind myself of what I learned and am less likely to make the error again.

This is usually what I do:
1. Play game
2. Run game through quick compute analysis to quickly see if I made any tactical blunders. If I did, I will read the computer's recommended line and try to play it over in my head.
3. If there weren't any obvious tactical blunders, then i look through the game carefully to see if there was a point where the momentum shifted from me to my opponent (e.g. in a recent game, neither side made a tactical blunder, but I made a positional blunder that allowed my opponent to obtain and use a Knight outpost).
4. I then determine what I messed up on and how I could fix it. If I do not know what the mistake was, then I suppose I could post the game on this forum or another chess forum and go over it with more experienced player. However, most of the time, I know what the error was.
Re: if you know where all your mistakes then how are you making them in a game.
The issue is that sometimes, I forget what I learned. So, I make the error. When I review the game, I then remind myself of what I learned and am less likely to make the error again.
Sometimes a temporary change in your playing environment may help. Say like playing people in park(if they do that in your city), randomly playing at other sites, play your friend of a friend(fresh opponent), cafe. Sometimes it your association with where you are playing that makes you make the action and over a period of time a habit.
A mental thing. Example whenever i get in a bad funk playing at chess.com i give it a break at chess.com and play else where. I can list a couple reasons why i think it works or maybe i am tricking my mind in doing something correctly.... bla bla bla. Not sure why it works but it does for me anyway.
I do use videos, but mostly for openings. e.g. I watched Aagaard's "Nimzo-Indian the easy way"