My favorite basic engineering reference is:
Stepniewski, W.Z. & Keys, C.N. Rotary - Wing Aerodynamics
NY, Dover, Trade PB EditionIncludes Vol. 1: Basic Theories of Rotor Aerodynamics and Vol. 2: Performance Prediction of Helicopters.
I have not seen Leishman's book, he is very bright, U of Maryland.
Try
www.abe.com for used book searches, about 2000 bookstores world wide.