Below are "recommended" reading lists for each FAA license through to CFII.  Purchase of all of these books in not mandatory.

Cloud 9 Helicopters stocks all of these materials, they can be mailed or shipped to prospective students on request.

Private License

Practical Test Standards (Private Pilot)

Rotorcraft Flying Handbook

R22, R44 or Schweizer Pilot Operating Handbook

ASA Test Prep (Private Pilot)

FAR/AIM

Principles of Helicopter Flight

Aviation Weather

Helicopter Oral Exam Guide

Private Oral Exam Guide

VFR Charts

Airport Facilities Directory

Rotating Plotter

E6B Computer

Kneeboard

Instrument Rating

Practical Test Standard (Instrument)

ASA Test Prep (Instrument)

Instrument Flying Handbook

Instrument Procedures Handbook

Aviation Weather

Aviation Weather Services

Instrument Oral Exam Guide

FAR/AIM

Approach Plates

VFR Charts

IFR Charts

Airport Facilities Directory

Kneeboard

Commercial License

Private License Books/Equipment

Practical Test Standards (Commercial)

ASA Test Prep (Commercial)

Commercial Oral Exam Guide

Additional Optional Books:

Jeppesen Instrument/Commercial

Cyclic & Collective

Certified Flight Instructor

Private License Books/Equipment

Commercial License Books/Equipment

Practical Test Standards (CFI)

ASA Test Prep (CFI)

Aviation Instructors Handbook

Certified Flight Instructor Instrumet

Instrument Rating Books/Equipment

Certified Flight Instructor Books/Equipment

ASA Test Prep (CFII)

Practical Test Standards (CFII)