My two main responses to this book, summarized: 
 1. "Wow, that's really fucked up." 
 2. "I love it." 
 This is a very, very strange novel - but it's totally worth muddling through the initial confusion. Carolyn and her 'siblings' were adopted and raised by an immensely powerful entity they call Father; now, like him, they are Librarians, each the master of his/her own 'catalog' of magic. Chaos ensues when Father goes missing. It's a continually unravelling mystery, with new bizarre elements added seemingly every page. 
 The worldbuilding is the best part 
 Without spoilers, here's a taste of what's in store for you in this book: the suburban undead; vengeful lions; mind-bending pseudoscientific explanations for magic; tentacled abominations of the deep; guns and helicopters; variations on the Book of Genesis and possibly Mesopotamian myth; parallel worlds that never were; uncountable resurrections of the dead. (Fair warning: along with all this delightful absurdity comes a good amount of gore and some depiction of sexual assault, so be wary if these are things that bother you.) 
 Scott Hawkins writes excellent prose: it's filled with wry humor, spare in just the right places, and masterful at foreshadowing. He's got a good ear for character psychology: the Librarians, who have had some of their humanity tortured away from them, have appropriately bizarre interpersonal relations. 
 My advice to readers of this book: keep reading. All the pieces, no matter how disjointed, fall into place eventually. And there's a twist you definitely won't see coming. 
 A few disappointments along the way 
 The Library at Mount Char hints at a vast and fantastic world, but we see so little of it! At times I appreciated the intentional gaps, written to be filled in by the reader's imagination - intentional omissions can deepen the sense of mystery - but ultimately I wanted much more magic than I was given. Tell me more about the other Library catalogs! Tell me about the powerful beings alluded to twice but never again! Sadly, the author indicates no interest in writing a sequel. I'll have to make do with the scraps of this magical world I was given, spare though they may be. 
 I wasn't fully satisfied with the character development, either. It doesn't make much sense for Carolyn to funnel all her anger at David, rather than saving the bulk of it for Father, the one who tortured the once-sweet boy into becoming a sadistic killing machine. 
 I really wanted to see much more of Carolyn's interactions with her 'siblings' - especially with Michael, her closest friend among them. I would gladly have traded some 'screen-time' of their strong platonic friendship for, say, half of the time spent with Erwin, an American war hero. (I didn't give two shits about Erwin. He was comedic but flat and very, very annoying.) 
 Despite these frustrations, I'd still recommend this book - particularly to any fantasy-lovers with a strong stomach for violence!