Plack Handbook
Link: Plack Handbook
Updated the popular Plack Advent Calendar 2009 and zipped it up as DRM-free EPUB and MOBI, best to be read on Kindle, Nook or iBooks. $5 to download the ebook format but the raw source code of the book is available under the Creative Commons license as well.
Go buy the book to support the development of Plack :)