Basically, a libriomancer can "reach into" any book and pull out the stuff described. So you could reach into Morte d'Arthur and pull out Excalibur. If your immediate thought on hearing this is anything other than "Gordon Bennett, that's the most overpowered thing I've ever heard", then you just haven't thought it through properly. Isaac, the protagonist, repeatedly and casually pulls healing potions and laser pistols out of books and uses them. It was discovered by Johannes Gutenberg, who is still alive because he pulled the Holy Grail out of the Bible and used it to make himself immortal. And yet... the world, Isaac's world, looks roughly like ours. That's ridiculous.

The book does falteringly attempt to set up some limitations. The physical book itself is the portal by which things are brought out, so they have to be smaller than the book; you can't pull a Challenger tank out of a Tom Clancy novel. Living beings can't be pulled out; written text doesn't describe them properly enough, and so the mind in the body isn't properly formed and they go mad. A book can be "locked" (by Gutenberg, who hasn't told anyone else how to do it) so things can't be pulled from it, which he's done on a whole bunch of titles such as the Bible and Lord of the Rings to stop some joker pulling out the One Ring. Magic mirrors and the like tend to focus on their native (fictional) world and not ours, so you can't pull out scrying devices that work here. Doing it too much gives you headaches and might destroy your mind entirely. A pulled thing should be returned to its book after some nebulous time limit; you can't keep it for ever. And the book has to be a published thing that many people have read: you can't just write "lightsabre" on a Post-It note and yank one into the real world.

But none of these limitations are show-stoppers. We could have a world in which there is limitless energy and luxury. With libriomancy you could make us into the Culture in about half an hour. And yet it hasn't happened. The basic explanation here is: Gutenberg is a dick. He wants -- insists -- that the Masquerade is preserved, that humanity can't know about magic, that it'd be a disaster, that we can't cope with the power. Reed Richards Is Useless, to a limitless degree. Apparently he's exceedingly worried about what a bad person would do with the magic, and that is a legitimate worry; a villain libriomancer or an idiot libriomancer is a global humanity extinction event, which is why you can't pull out the One Ring. But the amount of good that could be done with this power is breathtaking. No disease. No hunger. No death. And it's not allowed.

OK, I get his worry -- the "humanity just can't cope with the power" argument is a pretty long-standing one (see Michael Crichton's Sphere, for example). We're not mature enough as a species. Fine. But it makes the whole book feel like an anticlimax. Isaac spends a bunch of time farting around with Narnian healing elixirs and stopping vampires and all the time I'm thinking, why do we have to live in this crappy world rather than somewhere amazing? Libriomancy isn't used to its full potential in the book, because it would be basically impossible to write a book in which it was so used, and so it shouldn't be written about because any book that contains it is guaranteed to be anticlimactic. The sense of letdown and disappointment is volcanic. I don't think I'll read the rest of the series.