The main mystery of the book involves the surviving apprentices of one evil necromancer, and the search for a book that could give them god-like powers. But, as seems to be more and more the case as we make our way through the series, the real story is the cumulative effect of Harry's choices coming back to bite him. On the seventh book now, there have been more than a few. 
 While Blood Rites felt a bit like a character exhibition placeholder - where much was revealed but the book itself felt thin - Dead Beat made all that gathered information feel like it mattered. It's also the first time that I really got the sense that Harry is changing, and not just in the sense that he's getting more powerful. He's also becoming more corrupted, and more vulnerable to pushes from the baddies (psychologically speaking). 
 I'm interested to see where this all goes! 
 Best line: "Polka will never die!"