For some reason, I can happily listen to robotic voices for a very long period of time. I have listened to several books that way, when there isn't a good narrated version.

In fact, apart from a few quirks (a pause at the "." of Mr. for instance), the iOS TTS engine sounds almost pleasant to me. It's no where near a good narrator, of course, but good enough if there is no poetic value in the words.