I'd second Lessons in the Fundamentals of Go as a good buy. You'll get lot of mileage from it, and it gives enough of an overview so that you can figure out what other aspects of the game you need to work on.

I'm not sure quite sure which books you already have, but I'd recommend Attack and Defense and to a lesser extent, Get Strong at Attacking.

The former is a good theoretical overview of the middlegame, and the latter is a problem book of a number of full-board positions which show good continuations. I think I used the first book a lot more, but only bought the second when I started thinking to myself, "I know I should be able to attack that group and get something out of it, but how?"

I think the middlegame is the most important part of the game, when you are learning. Learning how to manage weak groups (or not make them in the first place) as well as learning how to attack your opponent's weak groups will get you a lot further in the game.