Assuming you are an intermediate player here is a list of books.
From cheapest to most expensive.
Ryan Fee's 6 Max NL Strategy Guide
by Ryan "fees" Fee
How to Beat No-Limit Hold 'Em 6-max Cash Games (http://skilledonlinepoker.com/nolimit.aspx) by Bill "bigbabyjesus" Vosti
Small Stakes No Limit Hold'em (http://smallstakesnolimitholdem.com/) by Ed Miller, Sunny Mehta and Matt Flynn
The Poker Puzzle (http://www.thepokerpuzzle.com/) by Oliver "Improva" Marx
Bobbo's Bible (http://www.leggopoker.com/forums/general-online-poker-discussion/bobbos-book-149.html) by Rob "BobboFitos" Eckstut
Let There be Range (http://dailyvariance.com/nlhe-book.html) by Tri "SlowHabit" Nguyen and Cole "CTS" South
Easy Game (http://www.balugapoker.com/books.html) by Andrew "BalugaWhale" Seidman
Memoirs of aejones (http://www.memoirsofaejones.com/) by Aaron "aejones" Jones