The Beginner's Guide to Search Engine Optimization
Moz.com provides a comprehensive guide for SEO which is very necessary for making your site available and easily accessible by people on the web and if you want your website to do well on search engines than SEO is an better solution.
SEO in easy language we can say it is technique of submitting and formatting your website in a manner which is much easier for search engines to crawl as well help finding people relevant information .
SEO is very popular among web Developers but majority of people fails to optimise their websites.SEO is an essential element within your website making process.
Here is an printable friendly version of SEO by moz which you can read and apply on your website. http://d2eeipcrcdle6.cloudfront.net/guides/Moz-The-Beginners-Guide-To-SEO.pdf
You can try out other things like Guide by Google for Optimising websites and other SEO techniques available on the internet.
And Here is the Freebie by google - http://www.google.com/webmasters/docs/search-engine-optimization-starter-guide.pdf