If I were to write a nonfiction book, one of my "marketing" strategies would be to create an informative subtitle that fully explains the purpose of the book. This would give me another way to draw a potential customer to my book page. "Hey, that's exactly what I'm trying to learn." Or "Hey, that sounds like a interesting thing to learn." It also gives me another opportunity to incorporate key search phrases. Fiction books just don't use subtitles like this. However, it's not out of the question to use a tagline. A nice 5 to 10 word description of exactly what your book is about could possibly add just that extra bit of oomph to get someone to click on your link. I'm not sure about the idea, though, because I don't see it done a lot. Why not? Would you consider it?