Ann Smarty has produced an inspired article of the Ultimate Collection of Online SEO Tools. All the resources she mentions are useful for search engine optimisation, are free, and are web based.
I think one of the best SEO books on the market is Planet Ocean’s Unfair Advantage Book on Winning the Search Engine Wars. The site is a bit hard sell, a bit cheap and nasty looking, but the materials are excellent quality, and I’m sure to review their monthly update.
There are many collections of tools on the web, but you might also want to take a look at, and it is worth evaluating to see which ones you feel suit your requirements best:
Probably best to open the blog to comments, and see what other recommendations you all come up with…