I have the HD manual, and it's well worth the price ($60). I also picked up "101 Twin Cam Projects" , and while a tad on the expensive side ($35- I had a gift card to a book store) it has some valuable information. It's not a service manual, but does a good job of outlining what is involved in a particular Twin Cam project. For instance, have you ever wondered "what is involved in changing out a set of cams?". This book won't tell you exactly how to do it, but will offer general step by step instructions, and give time saving tips..