Excellent set of vim pointers and tips.

free online book about vim.

Nice article explaining some cool things about vi

This tutorial introduces you both to Vim's built-in Cscope support, and to a set of maps that make searching more convenient.

This book was written after Vim 7 was released, thus it includes all the new features, such as spell checking and omni-completion. If you want to read about the latest features there is no other Vim book to chose from.