It is said that today openings are connected to middlegame plans and even with potential endings. But I can't find those middlegame plans, except for some occasional comments in opening books. In fact I feel lost about middlegame plans. What is the best way to learn the plans related to the openings in my repertoire?
Try taking a look at books that discuss the structures arising out of various openings. Andrew Soltis' Pawn Structure Chess and Mauricio Flores Rios' Chess Structures both provide good overviews of the appropriate pawn breaks, piece placements, and defensive resources. Of the two, I'd say Rios' book is the more rigorous and comprehensive.
It's certainly possible to learn this by examining master games, but it's a lot more time-consuming. If you're going to try this, I'd recommend anthology / memoir books by authors who specialize in the openings you like (Kasparov: KID, Bronstein: KG, etc.). I found Svetozar Gligoric's I Play Against the Pieces useful in this regard because he divides the games into sections by opening. Sadly, this last one's become a collector's item in the last 10 years (well, sad for you, anyway), so it's pricey at the moment.
I also strongly recommend John Watson's Mastering the Chess Openings series (4 volumes) as a reference. Each volume can be had in used condition for less than $15, so it's not an outrageous purchase, and Watson's approach is to explain in words what's going on in the variations, and their pros and cons. You may not use 80% of the material, but whatever relates to your opening will will be a goldmine, and is certainly worth the price.
Some publishers do an exceptional job of trying to convey opening theory with these strategic pointers mixed in. For specific openings, I can recommend Everyman Publishing's two series, Easy Guide to the ... (about 12 volumes, now out of print but available in used condition) and Starting Out: The ... (about 40 volumes available).
There are only two overarching plans in chess.
- The short term one is to play to attack the opponent's king in a mating attack.
- The long term one is to queen a pawn in an endgame.
Study the master level games that relate to your opening repotoire and learn the typical piece manuevers that support the two plans I mentioned above.