Note that ImageJ is constantly evolving so there is no single book on ImageJ that covers all functions, macros and plugins available. That being said, the ImageJ User Guide is available on the ImageJ website. The following are some of other resources available:

• ImageJ Documentation at the ImageJ website.
• ImageJ Macro Language at the ImageJ website.
• Reference to the Macro Functions at the ImageJ website.
• Fiji website

Books:

• Digital Image Processing with Java and ImageJ by W. Burger & M. J. Burge. Springer Verlag 2006.
• A Practical Approach to Medical Image Processing by E. Berry, Taylor & Francis 2007. Contains CD with windows version of IJ and sample images.
• Image Processing with ImageJ by José María Mateos-Pérez and Javier Pascau 2013.

Manuals/Tutorials:

• The Online Manual for the MBF-ImageJ collection by Tony Collins describes the collection of plugins available from the McMaster Biophotonics Facility website. (Many of those plugins are also available from other sites).

• W. Bailer Writing ImageJ PlugIns – A Tutorial. An introduction to writing plugins for ImageJ.