Each of them have the Look Inside facility, so I was able to browse through a few pages of each.
The Huffington Post Complete Guide to Blogging
The Huffington Post is listed at No. 1 at Technorati, so I guess they know a thing or two about blogging!
ProBlogger: Secrets for Blogging Your Way to a Six-Figure Income
I have a lot of respect for the authors Darren Rowse and Chris Garrett and am a subscriber to both of their newsletters.
Blogging Heroes: Interviews with 30 of the World's Top Bloggers
The title really says it all here. 30 of the world's top bloggers give their own experience and advice. You can see the list of who they are at Amazon, as well as getting a peek at some of the pages.
As I said, I've left it to be a surprise which one I get (maybe I'll get them all!!), though I think I lean towards the Huffington one as the most desirable.