Books on opening and middlegame plans

I'm an intermediate player (1650 KNSB rating) trying to improve my game. I like to play a variety of openings, but in the resulting positions, even when I have an objectively strong position, I often find myself without a concrete plan and all I can manage to do is trade down to a drawn endgame, especially against systems like the Slav.

I used to own only a few game collections and learned openings from theory databases, but obviously those don't show the reason behind the moves, so I bought "Mastering the Chess Openings" by Watson which was supposed to explain more abstract ideas. However, I was slightly disappointed by it because it did only point out these ideas through a few random example games per opening and because many openings that I frequently face (Scotch, French Advance, Scandinavian) were not covered at all. I considered purchasing a book on middlegame strategy, but most of them seem to cover mostly general concepts such as pawn structures, bad bishops, knight outposts etc that I'm already aware of, and I'm not sure if they will actually help me in planning.

What book(s) would you recommend to help me find more useful moves and constructive plans?

"The Amateur's Mind" by Silman is great for beginning to understand strategy. If you need a more basic starting out book, try Seirawan's "Winning Chess Strategies". Seirawan's book talks about positional features whereas Silman's talks about planning based on positional features.

Vladimir Vukovic's The Art of Attack in Chess is an interesting book on that subject.

"Modern chess is too much concerned with things like pawn structure.  Forget it.  Checkmate ends the game." - Nigel Short

Hi friend...Alexader Kotovs books are ideal for you..try it!

These are good choices.  There is also a less well-known book "Simple Chess" by Michael Stean which many people seem to like.  It is all about strategy and plans in the middle game.  Alternatively you could buy a well-annotated games collection 

I love reading threads like this where people mention their favorite chess books.

I have about the same rating as you and I also struggle finding the best material to improve my game and especially the planning part.

"Simple Chess" as mentioned before is a great book. It is divided into different strategic topics. The games are easy to understand and very instructive. A type of book that I use when I have like 30-45 mins time left in my schedule.

"Plan like a Grandmaster" and "Think like a Grandmaster" both have some more concrete chapters about planning in general. How to develop a plan etc. Chapters I try to read frequently to learn because many years of blitz, hobby and unstructured chess has given me a very bad way of thinking while playing!
Alternatives are without doubt "How to reassess your Chess" and "The Amateurs Mind".

When it comes to know the plan in different openings I think that books in the "starting out" series are a good but maybe expensive choice. They are very basic and only emphasizes the main ideas in each line, but still better to use than some good but to advanced book with 300 pages about e.g. sicilian dragon.
Find your favorite variations and ideas and play through a collection of games with that variation, only finding the similarities in the games and maybe see how a change in the moveorder affects the games. In my opinion you should not spend to much time on details/deep analysis of each game (according to your current rating). Better browse through a lot of games in less time.

Ok, this was just my opinion. I'm only rating 1620, so my advice is maybe to newbee.