Perl, The Next Generation. Sorry, I guess that expression is already overused.
This book is unusual in that it is a report on a work in progress. Current production Perl is still version 5, but this book gives you the skinny on the next major version, in the works for over a year and still in progress. Although new syntax is described, the caveat remains that Perl 6 is a work in progress, and everything could still be subject to change.
The book was written by members of the Perl 6 core development team. It gives an overview both of Perl 6, the language, and Parrot, the interpreter and virtual machine (including a virtual assembler) developed as part of the Perl 6 design strategy.
If you would like to volunteer to participate in the project, this book is a must read. If you want to know what kind of changes to expect when migrating your code to the new version, there is lots to discover. If you merely want to get a sneak peek at what the future will bring, there is more than enough information to satisfy.
For me, personally, it is actually a lot more than I wanted to get into. This is mostly because I am too busy to try learning something I may have to forget right away. Rather, my needs are more that, once the new version of Perl is implemented, I'll want a reference and porting instructions (and tools!) so that I can migrate to and be productive in the new Perl.