A few books that are on my bookshelf which I have found useful are:
1.
Search Engine Optimization for Dummies by Peter Kent. This one has a lot of very good information. The copyright is 2006, so I'm sure some things have changed a bit, but still a good read.
2.
Starting an Online Business for Dummies by Greg Holden (I haven't read this one in its entirety - just a couple of sections as needed so far).
3. I just purchased a book this week called
Ultimate Guide to Google Adwords . I saw in another forum that someone highly recommended this book. I haven't read it yet, but after briefly scanning it - it looks very informative.
4.
Start Your Own E-Business: Your Step-by-Step Guide to Success by Robert McGarvey and Melissa Campanelli. This one also includes interviews with other online companies.
I would also search various forums for whatever you're looking to find out. Or just visit the forums and pick up whatever information is being "served" for the day. A few good sites/forums that I know of are:
a. Search Engine Watch -
http://forums.searchenginewatch.com/
b. Site Pro News (also has a daily e-newsletter) -
http://www.sitepronews.com/
c. High Rankings.com -
http://www.highrankings.com/
Hope this helps!