It depends on what you are looking for.  The two best books I've read about the mental game are Dr Dean Hinitz's Focused For Bowling and Dr Erik Lasser, Fred Borden, and Jeri Edwards "Bowling - The Complete Handbook of Bowling Psychology."  Both are excellent for strategies to build a strong mental game complete with exercises.
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"I cannot change the direction of the wind but I can adjust my sails to reach my destination." Jimmy Dean
http://www.allbowling.com/journal/public.php?uid=156&leagueid=53

Bowling 200+. Written by Mike Aulby and Dave Ferraro. Very good book in my opinion. Goes over everything. It's a bit old though, copyright is 1989. The bowling ball section might not be too accurate now, but the others are good.
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Hammer, Brunswick, and Storm.

www.hammerbowling.com
www.brunswickbowling.com
www.stormbowling.com