PackT has the Unity book, Unity 4.x Game AI Programming, as part of their “Free Learning” program which provides a different free ebook every day. Subjects are all over the board with the occasional Unity book or related books.
PackT probably provides the broadest coverage of Unity books of all the publishers.
Sweet
If your digging through PackT unity books, have a look at some authors like Simon Jackson and Venita Pereira.
They have some belter 2D books that have some nice core learning to get to grips with.
I know that Will Golstone’s book was on there too during the free learning phase, and although deals with Unity 3.x its still worth a read as the ideas and concepts are relavent.
so theres a few there worth a look at least 
(kind of off topic sorry)
I’ve noticed that Mike Geigs Unity in 24 hours is getting an update in collaboration with Ben Tristem from Udemy fame too … coming soon.