according to the 'A******n T****y C****e' website they sell a book:
Aeroplane Performance, Planning & Loading for the Air Transport Pilot (B727)
Roxy, ATC does sell said book, but at the end of the day that's all it is - just a text book on the subject. I personally found it very hard going to just read the book cover to cover and get a grasp of the content.
AFT on the other hand provide a self study course, not just reference material. They guide you through the material and provide worked examples, revision questions, practise exams, tips and all the help (via email and over the phone) you could possibly need to pass the exam and have a sound understanding of the principles. AFT is the way to go and worth it's weight in gold!
ATC books look good on your library shelf and are great to refer back to later on once you havee passed the exams and are revising/studying for an interview etc.