The Hollows Insider
For a series guide, The Hollows Insider is really pretty cool. Rather than a bunch of dry facts, Harrison gives us a bunch of dry facts AND a story AND some hilariously subjective observations. Told from the perspective of a reporter who's investigating Rachel, The Hollows Insider documents his research, with journal entries, selections from the newspaper's files and archives, FIB and IS documents, and various other papers he's stolen or stumbled across. It's particularly funny because while Devin (the reporter) gets some of his facts right, he's quite obviously way off on many things, particularly regarding Rachel's character and goals.
I'll admit I skimmed through a lot of the fact sheets, but I did enjoy seeing Takata's lyrics and Al's comments in the demon spell books. Most of it wasn't anything particularly startling, just more detail on information that was in the books, but there were at least a couple of "holy crap, really?" revelations. I think my favorite part, though, was the list Devin compiled of Jenks's best curses, which included such gems as "damn it all to Disneyland" and "worthless as a pixy condom." (Since the facing page is a list of the names of Jenks's 54 children, the latter is apparently quite a strong one.)
This is definitely not a book you want to get as an introduction to the series. It contains spoilers for everything up to Pale Demon. But if you're already a fan, definitely check this book out - it has cool graphics, funny stories, and interesting facts that may just tide you over until A Perfect Blood comes out in March.