Gateoverflow has its own book: http://book.gateoverflow.in
The book has been compiled by experts and has clickable links on questions which refer to this site for additional answers. By this month's end, the latest version would be up. You could also order a hard copy.
As far as the "best book" goes, no book can beat this one as this has been handcrafted by 100s of contributors(correct me if I am wrong).