The FAA (Federal Aviation Administration - US) provides some nice reading material on their website, including a free PDF download called "Helicopter Flying Handbook" (see link below).
http://www.faa.gov/regulations_polic...ying_handbook/
Which is also available as a paperback version at Amazon:
Additionally I can recommend the following books, however they aren't specifically tailored to helicopter flying, but certainly very useful and interesting to any aviator.
The above books are also available as a free download at the FAA website - see following link:
Handbooks & Manuals
Always happy landings