Those Were the Days: An Unofficial History of the Beatles Apple Organization 1967-2002 by Stefan Granados (Highly recommended by beatlemoney.com)

The Beatles on Apple Records
by Ken Mansfield (Foreword), Bruce Spizer (Compiler)

The Longest Cocktail Party: An Insider's Diary of The Beatles Apple Corps. Ltd. by Richard Dilello

•beatles.com: Official home of The Beatles and Apple

•schomakers.com: The Complete Apple Records

•en.wikipedia.org: Apple Corps. info from Wikipedia

•rockmine.music.co.uk: The Beatles' Companies