1) The ABC of SEO (Paperback):   by David George

2) Search Engine Optimization by Richard John Jenkins

3) Search Engine Marketing, Inc. : Driving Search Traffic to Your Company's Web Site (Ibm Press) (Paperback)  by Mike Moran

4) SEO Book by  Professor J. Michael Steele The Wharton School -- University of Pennsylvania 

5) Search Engine Visibility (Paperback) by Shari Thurow 

6) Professional Search Engine Optimization with PHP: A Developer's Guide to SEO by Jaimie Sirovich and Cristian Darie