The book also gave us the Nightmare Fuel that Vorbis has spent an eternity in a hell of his own making. But why? Because he's in the desert, and he's absolutely alone. He knows it, there can be no doubt, that he is all alone. But he still hears the voice of Om. Because it's his own voice and always has been. The worst Heel Realization ever.
Everyone in the desert is alone because they believe they have to face it alone. Brutha's seen the desert, and he knows they're not alone, and he knows Om will be there with them. Except for Vorbis, so Brutha's there for him, the only one who needs him.
Fridge Brilliance: Om took the form of the tortoise because at the time all true belief that Omnians had was in the tortoise. That it moves. They didn't exactly believe in Om, but they still considered themselves his worshippers and thus gave him this form.
Fridge Brilliance: Brutha can't read, yet carrying around the contents of the Library causes some of the information written in them to "leak" into his awareness. This could mean that he's finally starting to grasp how writing works, but it could also mean that he's become host to an entirely-mental version of a "tomb of words", like the Post Office from Going Postal. He's not reading the books, they're talking to him—and he's listening, the same way he was noted to listen to others talk earlier in the book.