"The New Rules of Marketing & PR" by David Meerman Scott has just been published by John Wiley & Sons, Inc. The book is about the use of blogs, podcasts, viral marketing and online media. While not directed to law firms, there's a lot lawyers could learn from it. I highly recommend it as a way to get familiar with all of these.
The forward is by Robert Scoble, who while an employee of Microsoft, showed how even large organizations can covey compelling messages using video on the Web at minimal cost. While in the book, Scott acknowledges that Web video is still new (It's barely started for law firms.), he points out that it's a great way to convey information and leave the competition behind.