I am not sure about the best but i can recommend my own, http://www.back-links.org/All-in-one-free-ebooks.php
there are many sources to learn about seo, not only seo you can learn almost about any subject just by searching in google. check seobook.com you can get/learn about seo.
Brad Callen (creator of SEO Elite) has a free book called "Seo Made Easy". Do a Google search and you will find many places where you can download it from.
I would also suggest Brad Callen's SEO Made Easy. It gives a dolid basis for all beginners, and some great tips! Plus it's free.
Of the many books and guides that are out, remember many are now obsolete due to the ever changing SEO landscape. I would suggest that you start learning what the SEO elements of On-Page SEO is (http://open-site.org/Computers/Internet/Promotion) and the techniques of Off-Page SEO (Blogs, Forums, etc.). With this, you can use DP to look at the modern techniques that you will need today.