A Byte of Vim

An open content book (under the Creative Commons Attribution-Share Alike 3.0 licence) that can get you up and running. Available as online HTML and as a PDF.

The Vim Book

The first book about Vim by Steve Oualline, available as a PDF (HTML version no longer available). Covers Vim 5.

Learn Vimscript the Hard Way

Not really exclusively about Vimscript, Learn Vimscript the Hard Way is a book about customising Vim, primarily useful for people who are already quite comfortable with it. Available online with sources, under a restrictive (“All Rights Reserved”) licence.

O’Reilly’s Learning the vi and Vim Editors

A non-gratis book from O’Reilly and Associates (available as a paid Ebook download in several popular formats, or as a paper book) that covers both vi, with a focus on Vim.