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)