Well sorry for resurrecting this after so long, but I wanted to give an update on this in case anyone was still interested. After a very long time (and a lost shipment), I finally got my copy of the sketch book. After going through it, I've gotta say that I love pretty much everything that's in it; but let's get into what you guys are interested in. There are quite a few turtle pages in there, but most of them can be easily found on dan's
deviantart account. It's all basically inks of some finished comic pages and covers, that mini leo comic he did to get the job at IDW, and some rough sketches. The only thing that I believe we haven't seen before are some action poses (there's gotta be like 6 or 7 of them) that take out one page each. However, I don't believe that justifies the investment if you are just looking for never released turtle art.
Personally, I love the book so far; I haven't had the chance to really delve into it yet (exams... exams everywhere!), but from what I've seen after flipping through it, I can tell you that I'm really happy that I got it. There is a ton of stuff in there, including some notes from dan himself (btw, you can really tell nobody edited the thing; his redacting skills aren't that great), about how he went from amateur to pro. The book is entirely in B&W, and the pages aren't glossy; they rather feel like newspaper pages.
Anyway, hope this helps someone. Sorry for the delay (: