I actually sort of pieced together everything I could from Nathan through his (now discontinued) podcast, blog posts, and interviews.

I purchased Authority (just the book) afterwards, in case there were any other useful pieces of info in there and to say thanks for all of the free content that helped me so much, and just buying the book up front would've saved me a lot of hunting around, all the good stuff is in there :)

That being said, the podcasts and interviews are still well worth a listen, if only to really reinforce some of the more important ideas and build confidence in the approach by hearing about other folks' success stories.