I have yet to read the diary (that and the cassette tapes are the final holes in my TP collection) but I own both the autobiography and the town Access guide.
I agree that the Coop book is very erotic. I actually could not believe how sexually minded Coop was in his youth. But I thought the book was excellent, and it definitely filled in many gaps left by the series.
I found it interesting that Coop never seemed to be quite the perfect human being he is in the series until a few years before he goes to town. It certainly seems like it took many years to shape him into the "too perfect" character he became.
I did feel that some of the book was underwhelming and did not meet my expectations, especially the Pittsburgh incident and Windom's descent into madness. The show made me expect so much more.
Still, all in all, a good read. And I definitely consider it cannon before Fire Walk With Me. Cooper went to Deer Meadows, end of story.
The town access guide took me awhile to get into. I thought it was far too silly, not taking itself nearly serious enough. But then I got to thinking about how quirky many of the characters are that live in Twin Peaks, including the alleged writer of the piece. As prefaced in the beginning of the book, Andrew Packard's will left a sum of money to inexperienced writers to make a guide to the town. With that in mind, it has a much better read.
Also, taking place in the "future" of 1991, it gives some insight onto the future of the story, such as Pete Martell's aliveness.
Sometimes things can happen just like this. *snap*
J'ai une âme solitaire.