Well I have a few books on the opening, some on tactics, a couple on endgames. But what my collection lacks is a good book on stategic/positional play. I feel like I am rather weak in this area because often in quiet positions I have trouble coming up with a plan or just improving my position. I tend to play for tactical positions but things don't always go that way and I need to learn to play more positionally aswell. I am looking for a book that targets intermediate and advanced players as I already know the basics of strategy. Any ideas?

You've seen Silman's Reassess Your Chess?

Going by your rating this book is specifically targetting for you, it focuses on developing plans against a wide variety of imbalances that occur in the middlegame and should shed light onto your play.

Chess Strategy for the Tournament Player, by Alburt and Palatnik. 

I found it helpful, but not too advanced to understand.  Some of the material was pretty tough for me though: I plan on going over it again in a while when I think I can grasp it better.

Thanks for the replies. I haven't read 'Reassess your chess' yet but I have heard alot of good things about it. I have also heard good stuff about 'My system' by Nimzowitsch, maybe I will consider getting both of them. Theres also a strategy book by Dvoretsky but it might be a little advanced for me, I am not sure. What do you guys think?

In addition, Logical Chess by Irving Chernev and Zurich 1953 by David Bronstein.