Hello,
We have not created any official books on UE4 yet, but and (the people who wrote the Mastering Unreal series) wrote our official online documentation. The following link will take you to their “Getting Started” guide:
https://docs.unrealengine.com/latest/INT/GettingStarted/index.html
We also provide a large assortment of tutorials at these locations:
Wiki:
Forums:
https://forums.unrealengine.com/forumdisplay.php?12-Community-Content-Tools-and-Tutorials
Youtube:
I hope that this helps, please let me know if you have any other questions.
Cheers!