I suggest Chess Strategy for Club Players by Grooten.
What Chess book to buy?

I suggest Chess Strategy for Club Players by Grooten.
Hear, Hear, I purchased this 'ebook' recently. A lovey tome of information. The chapter on open files were well thought out, and explained nicely.

There are so many books to chose from, most are well written, some authors are better than others at expressing their thoughts and ideas. Honestly, the best way to get better at anything is to find like minded individuals with common goals, and work to improve not only yourself, but those around you.
It's a bit late, but I thought I'd throw out an alternative path.
Don't get a book targetting some area of the game, but go for a tournament book, or a book on your favourite player. Some of these books can be classics of instruction too, and you get the feeling that you connecting with some great minds of the past.
Some recommendations that come off the top of my mind: Life and Games of Mikhail Tal, Zürich 1953 (annotated by Bronstein), My 60 Memorable Games (Fischer).

Get Silman's Reassess Your Chess and work through grandmaster level games (Gillbod above) with the principles he covers. Sometimes my son and I train by setting up a board and slowly coming up with candidate moves and then checking our analysis with the game move. I've bought literally hundreds of chess books over the past ten years, but I keep coming back to Silman and a database or an annotated games collection book (preferably). It helped me get out of the mindset that I had just lost/won a game because of a tactic. You will learn what those principles look like in action, the history of various openings, and your idea of what is possible will expand under the sheer genius of the players you are studying.
**Chess Mentor, here on chess.com, has a number of well-chosen lessons created by Silman that are wonderfully instructive. You'll just run out of them at some point.
Me's seein' tha', cool.