Wikipedia defines SEO as:

Search engine optimization (SEO) is the process of improving the volume or quality of traffic to a web site from search engines via “natural” (”organic” or “algorithmic”) search results. Typically, the earlier a site appears in the search results list, the more visitors it will receive from the search engine. SEO may target different kinds of search, including image search, local search, and industry-specific vertical search engines. This gives a web site web presence.

Here I have collected some free pdf books about SEO. Download, read, implement and enjoy.

• Official Google’s SEO Starter Guide: ( PDF )
  
• Google Adwords Secrets by SEOBook ( PDF)
• SEO for WordPress blogs by Blizzard Internet ( PDF )
• The Basics of Search Engine Optimisation (SEO). SEO Book ( PDF )
• Search Engine Optimization by easywebtutorials.com ( PDF )