Unity is popular, and only getting more so. The fact you can currently get the iOS and Android Basic versions for free is only helping that fact. More and more people are also getting in on the publishing side of things and there are already over a dozen books available.
I put together this guide to help make the process a bit easier. It lists all of the current and upcoming Unity books on a single page. It contains all the applicable information you might need ( Publisher, number of pages, average rating, table of contents, authors description, links, publish date, etc ) to help pick out what book to go with.
So, if you are getting into Unity and are trying to figure out what book to buy, check it out. Also, I am trying to keep things as accurate as possible, so if a new book is coming out, or I missed an existing book, please let me know and I will add.