This article is from the Go FAQ, by Morten PAHLE firstname.lastname@example.org with numerous contributions by others.
First look at section 4.2 to find out which parts of your game need to
be improved. Then find a book which deals with this.
On his website, David CARLTON maintains an excellent online
bibliography of all the English Go books available, sorted by subject,
player strength, and publisher. His site can be found at
If English is not your language, his site also links to similar sites
which deal with non-English books.