Of all the books and research we've read, these are the ones we've found the most useful information from and influenced a large part of our redesign at Wufoo :

Call to Action http://www.calltoactionbook.com/ (how to think about landing pages and navigation)

Landing Page Optimization http://landingpageoptimizationbook.com/ (best technical book on how to measure landing pages, do not buy SEO books, they're all terrible and rehash what you see on most blogs)

Word of Mouth Marketing http://wordofmouthbook.com/ (sometimes great marketing has nothing to do with what you do on the web)

Made to Stick http://www.madetostick.com/ (just a great read on how you should shape your copy and ideas so they have maximum sticking power)