I haven't tried it for book promotion. I have however used it to promote websites that I run, my business, and currently, my upcoming workshop. The impressions are amazing. The clicks, a little less so. But I know that in order to build my brand people must see my name/company name at least 9 times before it starts to stick and to do that, I need the impressions.
For what it's worth, I'm also trying Facebook ads and those I haven't been as impressed with. Facebook just has a smaller audience than Google. So the impressions have been less, and so have the clicks.