Really, I'm going. But this looks good, so I thought I'd plug it. One book that I've recommended to friends and students since forever is Andy Soltis's Pawn Structure Chess, and the newer, most sophisticated book by Mauricio Flores Rios called Chess Structures: A Grandmaster Guide is another book I regularly recommend. IM Sam Collins' six-volume DVD set, linked to above, is on a big discount (less than 40 USD) and seems to me a work along the same lines as the books mentioned above. I just bought it myself and recommended it to several of my students, so there's not even a whiff of an ulterior motive. (For those of you who remember my ChessBase shows, that was nine years ago. I've had no professional relationship with the company since then.)
Check it out, and see what you think. The reason I think the works linked to above are valuable is that pawn structures and the middlegame themes that arise in those structures recur, and they appear across different openings (and often with reversed colors). It's not enough to "know" your openings, in the sense of memorizing various sequences of moves that end with the desired chess punctuation mark; one has to know what to do when the middlegame rolls around. One very good but undervalued way of doing that is to look at tons of games that start at or near the place where your opening analysis ends, but another is to look at typical games based on the resultant structure. This is valuable even if the position isn't identical to anything in your prep. It might even be from a different opening, but it may not matter that much. Understanding common plans, maneuvers, pawn breaks, and typical endings will give you a big advantage against a player who only knows the same "official" theory that you know.