The God Machine (Hellboy) by Thomas Sniegoski.  I picked this up on a whim via Bookmooch, after having read Sniegoski's wonderful collaborative series with Christopher Golden, The Menangerie, still not expecting an awful lot from a tie-in novel.  The good news is there's quite a lot to be pleasantly surprised by.  I count myself among the many casual Hellboy fans - I dig Mignola's art style and find the blend of humor, heroism, and arcane obscure references to literature hard to resist.  Even so, it had been awhile since I'd picked up even a B.P.R.D comic, and even longer since I'd read the start of the Hellboy saga. 

Sniegoski perfectly captures the tone of the characters, giving an ideal blend of background to keep new readers informed while old ones smile nostalgically.  One of the other good things about this is that it's a well developed "case" of its own in keeping with what we've seen of the rest of the Bureau's investigations - the titular machine is the working of an old one, driving the fervour of a mechanical cult to steal objects imbued with religious significance.  Action, suspense, the big red guy himself, and a cup that Elvis himself sipped from.  As tie-ins go, this one's top notch.