The Humble Little Ruby Book – A nicely-written free book on Ruby, by Jeremy McAnally

The Little Book of Ruby – An 87-page free introduction covering Ruby basics.

The Book of Ruby – A 400-page free Ruby bible by Huw Collingbourne.

Programming Ruby – The first edition of the so-called Pickaxe book, available for free, by Dave Thomas, Chad Fowler and Andy Hunt

Why’s (poignant) guide to Ruby – The legendary book by why the lucky stiff. With foxes, little elves, and… chunky bacon!). Exquisitely weird and witty.

Read Ruby 1.9 – “Very early draft of a book about version 1.9 of the Ruby programming language, released under a Creative Commons license.”

Learn Ruby The Hard Way – A Ruby translation of the popular book Learn Python The Hard Way, by Zed Shaw.