However, thanks to JP's tireless adjustment of margins, there is now a book you can get for free as well. It's a nicely typeset version of the web-based documentation for Twisted's core, with a paginated table of contents, which you can print out and read on paper, if that's your thing. This book is 255 pages - and it only covers the core portions of Twisted.
At some point I'm going to try to put together an "extended mix" version of this book, that includes documentation of the less well-documented and stable parts of twisted. For now, though, I'll trust it'll take you all a little while to get through the first 250 pages :).