Yep. I like the OP's attitude. First, keep it fun, especially if it is a hobby, and not a profession!

I won several games memorizing the ins and outs of the Fried Liver Attack. Why? Because I found it fun to study this romantic opening, and to swindle some opponents who didn't know the theory.

I would suggest to the OP to study a few ins and out of the Kings Gambit (1.e4 e5 2.f4). It's a blast to study, and a ton of fun to play. The FCO (Fundamental Chess Openings) is one book that covers all of the ins and outs of this great, romantic and historic opening.

Chessgames.com is a great sight to find all of the really old romantic games, and to see how people got swindled in the Fried Liver, King's Gambit, and other old openings. If you look at grandmaster games, these old openings don't look fun, they even look drawish. But don't let that disappoint. My King's Gambit and Fried Liver Attack games don't look anything like those games, since they are full of crazy blunders, from both sides, as both openings are tricky to play - and I learn some cool tactics and checkmates along the way - which are also things that keep chess fun for me. 

I know that. Perhaps my sequence of sentences mixed things up a bit. The point I was trying to make in that paragraph was something I got from Danny Rensch - that is, to pay close attention to the pawn structures - even taking all of the pieces off of a study board, so that you only see the pawns. That's what Danny does in his lectures, and it helps one to see the openings in a more meaningful way, and helps to answer the OP's question - why this move, and not that move.

Basically, the pawn structures define the optimum piece placement, or something like that.

There is the ideal pawn center for White, e4 and d4, and then there is the ideal extended pawn center for White (not as easy to support!) - c4,d4,e4 and f4.

Example - If Black plays the Pirc Defense, not wishing to contest the center by placing pawns in the center, then it makes sense for White to be more agressive in the center with the Austrian Attack, not that this opening is the only good way to meet the Pirc. Black intends to contest Whites large pawn center from the flanks, among other things, with moves such as Bg7.

Before looking at the theory, a good question to ask oneself is, what pawn break should Black attempt - c5 or e5 - and why? Pawn breaks are really good things to study, and can be fun to study. Dan Heisman talks about pawn breaks a lot, and all of his articles are free to read at ChessCafe under Novice Nook. A treasure of chess information for beginner to stuff many advanced players still struggle with (Class A, Expert), so don't let the word Novice Nook fool you.

It's a very complicated question. Rather than give you a plat answer, I suggest you borrow or buy a) Garry Kasparov's "On Modern Chess -- Revolution in the 70s" Part One. The reason for saying that is because it gives a great look into how GMs develop opening ideas and how theory develops, which eventually percolates down to our level.

For example, Garry discusses several tabiya in the Scheveningen.

b) borrow or buy the four volume series "Mastering the Chess Openings" by John Watson. (I think volume 3 - dedicated to the ideas of the English Opening - is the best volume in the set.)

Watson discusses many of the typical strategic ideas of various opening set ups and also discusses general issues about opening play.

A couple more titles worthy of attention are "How to Open a Chess Game", which is perhaps tricky to get hold of (it's out of print) but worth searching for because 7 GMs (eg. Bent Larsen and Paul Keres) discuss opening questions; and "Secrets of Opening Preparation" by Dvoretsky and Yusupov, esp. the lecture by Dvoretsky where he discusses studying the historical development of an opening by looking at many games over the decades in a particular variation. That approach by Dvoretsky will help teach you the opening you wish to play but also give a deeper understanding of why some lines have faded away -- they are refuted or plans are developed casting doubt on the set up -- whilst others come to the fore.

A very good, practical aid is the New in Chess Yearbook series. Each of the four yearly issues is packed with surveys on all sorts of openings where the survey author discusses current GM praxis, gives a selection of annotated games in that opening for study and - often the most informative part - discusses typical ideas/structures in that opening under discussion.

So, there is a plethora of very good material which can help you get a handle on openings.

You are quite incorrect on this point.

While it is true that an isolated pawn may occur in many different situations, the Isolated Queen Pawn is by far the most common and the most theoretically significant.  More than a century of research and practice by the greatest minds in chess have developed the understanding of this position - nobody works on isolated g-pawns.

The IQP is a proven advantage in the middlegame where its owner can utilize the e5 or c5 squares effectively, and/or those files.  In the ending, it can be vulnerable but not necessarily fatally so, so in balance it cannot be said the IQP is truly a weakness unless it is blockaded and attacked.

Learning how to play with and against the IQP is fundamental to positional understanding in chess.

The John Watson books sound interesting, especially the 1.c4 as I've been sticking to the English as my white opening for now (English as white, KID against d4 and Sicilian against e4 as black).

I have enjoyed many videos on chess.com. Some of them I find very instructive other ones I find to be less instructive because they are above my level so I'm not absorbing much. Some of GM Shanklands latest have been more specific on each of the moves (even if at times he does sound like he's in pain having to explain why a move is so easy). I have been watching Rensch's Pawn Structures 101. I don't think I'm absorbing all of the information available in those yet so I'll have to go back over them as I get a bit better. Part of that might be also that for the most part the ones I've watched so far are openings I don't play so it makes it harder.

But that is a book series, not one book on the English. Sure, a person can purchase a multi-volume set on one opening, but I don't think that is what the OP is after, reading his first post. I guess it depends on your skill level and how much information you want.

For me, one book on the English is plenty, and I happen to like John Watson's writing style. I am not an International Master!

I would argue that the ambitious player should puchase Marin and Watson if he wants to play the English!

Marin's rep book is limited to the 1.c4/2. g3 move order -- notably to avoid any possible transpositions into the Hedgehog formation (which Marin loves as black by the way). Watson doesn't cover every possibility in his volumes -- Musikmole describes quite well JW's intention with the 4 volume series -- but he does look at the 1.c4 and then 2. Nf3 and 2. Nc3 options, which can give rise to variations/systems not covered by Marin.

Incidentally, there is an old piece of advice which I first saw mentioned by Kotov in "Think Like a Grandmaster", although he probably isn't its originator. It is: study/learn a few systems in great depth whilst having a general knowledge of all the openings. Even if one never plays the Benko Gambit with White or Black, for example, it will enrich one's knowledge of chess to be aware of the typical ideas of the Benko. Did you know for example that Black, even though a pawn down, often can afford a queen exchange because it increases his positional pressure for the gambitted pawn? (It's a big surprise when you see that idea for the first time.)

I was adviced not to take up the Sicilian at my rating level, because these moves black makes - I cannot understand them either.