I recommend Yasser Seirawan's books. They are accessible and do what they say on the cover: Openings, Endings, Combinations etc.
I also picked up Neil McDonalds new book at the library this week: "The Art of Planning in Chess" and it's excellent.
This thread is so useful I wonder if a book recommendations and reviews section could be included somewhere on the site. Maybe even a supplier could sponser it and offer a discount.