There's a thread on this over at the bay12 forums, this post is by the author.

Quote Originally Posted by Splynter View Post
What on earth is a real time book? The ones I read are turn based.

Seriously though, what the hell does that mean?
To answer your question Splynter:

"This book represents an experiment by O'Reilly in "real time" publishing. This means the book is likely to get frequent updates - I believe that people who buy the book will actually get updates pushed to them as the book is maintained on an ongoing basis by me (and possibly others). I love being part of an experiment, makes me feel dwarfy."

I'm guessing that's in reference to an e-book version as I can't see them making money mailing out new pages and chapters as they're done.