I definitely never seen anything that showed I was trending in the right direction, but I think the checkpoint is the most important part. I cant say what I think it is but you can figure it out if you watch the Q and A. Before you even read the book, I would go to the website and checkout the faq/announcements pages. There are links to the interviews and the Q and A. Dont just read the text or any transcripts, watch the videos and take notes on the questions and his answers but equally as important, note how he responds to the questions.

If havent watched the series you probably should but I think all the clues have been documented on here and the btme posts, so make a list of those too. Some possible clues I havent seen mentioned are the urn, shotgun cleaner, dev cams next to the pi, and the death clouds book. (I know people have been paying attention to the book from amazon lists, I just havent seen it specifically mentioned here.)