The New Rules of Marketing and PR: How to Use Social Media, Blogs, News Releases, Online Video, and Viral Marketing to Reach Buyers Directly, 2nd Edition
By David Meerman Scott
Summary: A great overview of grassroots marketing, recently updated. Highly recommended if you are just getting started with marketing.
Always Be Testing: The Complete Guide to Google Website Optimizer
By Bryan Eisenberg, John Quarto-vonTivadar
Summary: A must-have if you want to do split-testing. Lots of great concrete ideas, plus some good marketing gems.
Duct Tape Marketing: The World's Most Practical Small Business Marketing Guide
By John Jantsch
Summary: Great general book - probably the first marketing book I read.
Search Engine Optimization: An Hour a Day
By Jennifer Grappone, Gradiva Couzin
Good overview of SEO, with suggestions for small steps you can implement over time.
Purple Cow: Transform Your Business by Being Remarkable
By Seth Godin
You need a product that stands out in the crowd. Seth Godin's books are all useful and entertaining.
eBoot Camp: Proven Internet Marketing Techniques to Grow Your Business
By Corey Perlman
Summary Another good beginner, overview book about marketing on the Internet
Permission Marketing : Turning Strangers Into Friends And Friends Into Customers
Seth Godin
Summary: This is really useful for understanding why you need a list.
Trust Agents: Using the Web to Build Influence, Improve Reputation, and Earn Trust
By Chris Brogan, Julien Smith