Here’s a good intro tutorial on Quaternions for Unity3D from Unity Gems. It comes w/ a sample .unitypackage.
For a more in depth understanding, the following two books might help.
I liked the Quaternions section in the 1st edition because it gave you more context by going over Complex Numbers and performing addition, subtraction, multiplication on them first:
3D Math Primer for Graphics and Game Development
By Fletcher Dunn and Ian Parberry
Jones & Bartlett Learning
I’ve looked at the 1st edition of this book for an overview of Quaternions and it was fairly straight forward but no overview of Complex Numbers:
Essential Mathematics for Games and Interactive Applications: A Programmer’s Guide
By James M. Van Verth, Lars M. Bishop
CRC Press (Taylor & Francis US)