What I read, I liked. XML-RPC is fairly straightforward and simple stuff when you get right down to it. The information and examples were both friendly and useful.
I think that this book could be heavily expanded to include chapters on topics that were mentioned in Chapter 8. Specifically, logging, authentication and especially payloads (ssl, etc). A chapter that deals specifically with Apache/mod_perl could also be added, as it is brought up a couple of times (p.65,88).
Specifically, I'd like to see a setup (sample client and server) for XML-RPC using https.
Medium nit - The current release of Frontier::RPC that's out on CPAN is 0.06. I found 0.07b3 off of Ken MacLeod's home page. So, 0.07 is BETA. Might want to mention that.
Very, very minor nit - Page 71, first large paragraph - add the word "respectively" to the end of the 3rd sentence (after 'object').