REBOL: The Official Guide
"REBOL reminds me of the little engine that could. It's ridiculously small, the entire language is a sub 400K download, but oh my, the power it packs into every last byte, it's almost scary." [Turk's Head Review]

REBOL Press
Official site for REBOL publications. Includes sample chapters for several of their books.

REBOL for Dummies
Maintained by author Ralph Roberts.

REBOL Language Documentation
All the official documentation from REBOL Technologies.

REBOL Essentials
Creating lightweight cross-platform applications by Viktor Pavlu (PDF file)