Want a hard copy? Buy via Lulu (Bound paperback, B&W)
Many resources are supplied, including files with sample mouse modifiers, track templates, custom toolbars and much more. Its sample work files and dozens of step by step tutorials will lead you to an understanding of how you can use these features in combination with each other to make REAPER behave for you the way you want it to.
Although not light on theory, it has a definite practical emphasis, with links to archives containing some 40 or so project files, with step by step examples. These help you put your knowledge into practice.