While I was at RubyConf, I picked up a copy of an Addison-Wesley shortcut on Mongrel by Zed Shaw and Matt Pelletier. This one is a bit longer than Rubyisms in Rails, and costs a bit more ($15). It is every bit as good though, and well worth the small investment.
One thing surprised me about this book. I'd assumed that Rubyisms in Rails was produced in a landscape format because of its heritage as a presentation, but Mongrel is laid out the same way. I guess this is a design decision by Addison-Wesley, but I'm not sure why they went this way.
My favorite part of the book is the inclusion of Zed Shaw's opinions on programming. They're (mostly) bundled into little sections called "Zed Says", and they're gems! I also appreciated the (repeated) warning that deploying an app into production is not something you're going to do in a day — not if you want to do it right. I've been a systems engineer for a long time, and I'm amazed at how many people forget about everything outside of the codebase when it comes to making a web-based application work.
Mongrel users are also going to appreciate the information about making it work with a variety of web servers, extending it to do more stuff, and generally making it do all the tricks you're looking for in an application server.
If you hang around the mongrel (or Ruby or Rails) community for a while, you'll soon realize that Zed can swear like a sailor. If that worries you, rest assured, this shortcut is fairly tame (you'll read worse language on Tim Bray's web page) — it's like Zed said in his RubyConf presentation, "swearing is so 2005".
So, what are you still hanging out here for? Go grab the mongrel shortcut and get reading.