The two 'must have' books are Shawn Coyle's Cyclic & Collective (excellent and much discussed previously on PPRuNe) and newly published, Fatal Traps for Helicopter Pilots by Greg Whyte.
Fatal Traps is a thorough analysis of all types of rotary accidents. Mine has just arrived and looks a very useful read. You can order it direct from
www.fataltraps.com. Good service and it was with me in about a week from NZ.