Learning Unity 2D Game Development by Example written by Venita Pereira published by Packt Publishing is quite a wonderful introduction to not only Unity itself but the new 2D game development suite it recently added. I had been meaning to checkout the 2D Unity mode myself for quite some time, and this book gave me a proper introduction.
After being a major supporter of Construct 2, mainly due to how easy it is to approach for new comers and the fully featured sprite editors, I can now easily recommend Unity as well.
Read on to find out why.