Twisted Network Programming Essentials by Jessica McKellar & Abe Fettig (O'Reilly Media) gives an introduction to Twisted, a Python framework devoted to event-driven programming, and particularly it's application to networking. The book covers both high level general concepts of Twisted, as well as detailed examples covering some of the protocols Twisted supports, including my particular interests of HTTP and SSH.
In common with many technical books, things start slowly with a chapter on installation which (IMO) would have been better relegated to an Appendix. I like a technical book to start with motivating examples followed by an overview of the technology, and that's exactly how the second chapter 'Building Basic Clients and Servers' works - it is a really good introduction, describing Twisted's core architectural separation of Transports, Protocols, and the Reactor, with some solid introductory examples (including the obligatory echo server).
The next chapter, 'Writing Asynchronous Code with Deferreds', is slightly disappointing. According to the preface, this is a new chapter in the second edition (I've not read the first), and it certainly describes 'How' to use Deferreds, but I would have appreciated more on the 'Why' - the examples are contrived demonstrations of how things work, rather than demonstrating any real use. The chapter opens by stating 'Practice will help you develop an intuition for how to structure asynchronous code', which is undoubtedly true. But this chapter won't - and I'm not sure whether it's the book or Twisted that's at fault here. Again, maybe an appendix would have been more appropriate for this content, so the use could be seen in realistic examples first?
The remainder of the book covers a variety of protocols (HTTP, Mail, IRC, SSH) and various deployment and best practices, including authentication, integration with databases, and concurrency. The section on deployment was particularly useful, and I enjoyed learning about (and using) the range of features available 'out of the box' in the twistd program. The use of Twisted's 'manhole' functionality to provide Python shell access to a remote program over SSH was also a highlight.
If you already know you are going to be using Twisted, but feel slightly apprehensive about it, I think this is an excellent place to start.