the reverse engineering framework
This book was released together with radare 1.0. Most of the concepts are explained there, some of the examples may not be compatible with radare2.
Recently, maijin started a project to update the radare book for radare2:
radare2 api (aka libr) is described in vapi files. Those files are translated by valaswig into swig interface files to generate the bindings for python, ruby, perl and others.
Some e-zines has published articles about how to use radare in some specific use cases.
Many articles are published in the examples in order to explain specific features of r2 with practical use cases. We are open to accept submissions of new articles
You may find slides, videos and audios of many talks about the features, capabilities and practical examples of using r2 for different tasks.