Some questions, which may be appropriate:
1) Which type of game(s) ought I be playing?
Ideally OTB (over the board) tournament games with time controls at least 60 minutes for each side... but ok, you're not looking to be world champ right 
so it's up to you. Keep it fun! But also the slower the better. Slow time controls are good because you need time to attempt to work new knowledge and skills into your game. Speed games are more about using the ideas and patters you're already very familiar with.
Is it impossible to improve only playing blitz? No. Can blitz be good for trying out new openings? Sure. But in general the longer the better if what you want is improvement.
2) Is it helpful to read books at this stage? If so, any suggestions?
An annotated game collection book or tactics book would be best IMO.
Game collection:
https://www.amazon.com/Masters-Chessboard-Modern-chess-masterpieces/dp/0713457457
Tactics:
https://www.amazon.com/Back-Basics-Tactics-ChessCafe-Chess/dp/1888690348
3) What's the best way to learn from a game, ie specific habits for making the most of a win or loss?
What I used to do was highlight my 2 biggest mistakes, (I would literally print out the PGN and get a highlighter lol) and save the game for future review. Once you have about 10 games, see if any of the mistakes are the same type. For example maybe you're often missing your opponent's knight forks. Then you can strategize on how you might solve this. Maybe notice that two pieces on opposite colors can't be forked by a knight.
Also pay attention to any positions where you were really confused for what move to make. Ask other players, an engine, reference materials, or something else, but try to find an answer to those positions. You'll notice some common solutions like "improve your worst placed piece when you're not sure what to do."
4) Is one's rating really the best way to analyze the strength/weakness of one's games? If not, what other indicators give one a good sense of where one stands and where to go from 'here?'
Ratings definitely don't tell the whole story. Some basic areas of study:
Openings, strategy, tactics, endgames
There are also basic skills like calculation and time management. Many different combinations can bring players to the same rating.
That being said, in general ratings below 1600 or so should be sure not to neglect tactics. In general you can focus on your weakest area when you want to improve, but some base level of tactical ability is foundational and necessary, so going for that first is never bad.
5) What is a reasonable goal for a beginning player with zero previous experience?
Lose 100 slow games (meaning not blitz or bullet). Haha, seriously, that would be my first goal for a totally new player
Or less negatively I suppose, just play 100 games. In the beginning you have to play to get a feel for how the pieces interact and get used to trying to visualize moves.
As for rating, some people will probably say this is too slow, but because you're not a kid with 8 hours a day to spend on chess, I will say 100 points a year. Would you be able to do more? Sure, but I wouldn't want to suggest something really hard and then have you be disappointed if you fail.
In general the "secret" to improvement is enjoying playing and learning to the point you're having fun doing it every day. If you can do this, then after years you'll have improved to a level you can't imagine playing at today
Thank you very much for your thoughts,
Hayes
Hello. I have been playing chess for about 10 months. I love it (and Chess.com), but being middle-aged and having never played before, I have no misconceptions about becoming "great" (or even good, for that matter). But, I would like to be progressive and purposeful in my strategy and tactics.
My present rating is a measly (~) 620 in Blitz and (~) 1100 in Daily, but the only game plan I know for improving is continuing to practice Tactics, present rating (~) 725, and playing games. What I'd appreciate feedback on is forming a game plan (not just a game "wing-it") for improvement.
Some questions, which may be appropriate:
1) Which type of game(s) ought I be playing?
2) Is it helpful to read books at this stage? If so, any suggestions?
3) What's the best way to learn from a game, ie specific habits for making the most of a win or loss?
4) Is one's rating really the best way to analyze the strength/weakness of one's games? If not, what other indicators give one a good sense of where one stands and where to go from 'here?'
5) What is a reasonable goal for a beginning player with zero previous experience?
Thank you very much for your thoughts,
Hayes