Is Pro Django book still relevant?
Yes, it's definitely still relevant. Although a lot has changed in Django since version 1, the internal parts and the concepts that Pro Django deals with are mostly the same. I'd have no hesitation in recommending the book - it's a really useful insight into how Django works and teaches some very useful methods as well.
I want to dig deeper into Django's internals and the official online documentation only goes so far toward that end. 
The reviews for Marty Alchin's Pro Django are fantastic (I've read Pro Python and really enjoyed it). However, the book is from 2008 and is based on Django v1.0.  My company builds off v1.3. 
Is this book still worthwhile?  How can I best learn about Django's meta-programming (beyond looking through the source code, which I am also doing)?
Thanks