Wheat's Bass Book, now in its 5th incarnation, is quite simply, the best bass resource on the internet. The author, James Martin, is a graphical designer and it shows in the clarity and simplicity of his layout.
The book is well thought out and has a logical progression of chapters in three sections. The first is mainly on technique and how to play, the second on theory and the third includes miscellaneous topics such as rehearsing with a band and reading music.
Unlike books made out of dead trees, Wheat's book has continued to evolve and improve from the day when it first came out.
I cannot recommend this resource highly enough.