Have you seen the Disney XD show Gravity Falls, created by Alex Hirsch? If you haven’t, you should! And you should probably do it before you read this book, because it’s a tie-in that will have major spoilers for the show–I think it will generally work better watching the show first, and then reading the book. Here’s a review of the show.
OK, so now you’ve caught up on the show, right? So this is that journal, the one that Dipper discovers and uses a guide to the town of Gravity Falls through the duration of the series, and the author of which is a major mystery of the show.