I'm biased but I always recommend that anyone interested in SEO start with the Beginner's Guide. It is a fantastic resource that is updated regularly frequently and I find it easier for folks to grasp than many of the alternatives.

It's not "straight from the horse's mouth" as is the case with the Google guide (which frankly what Google wants to drive rankings does not always tie up nicely with what actually seems to drive rankings in practice), but I always have users start there and then look further afield if they're still interested.