I highly recommend Bruce Emery's book and I just received an e-mail from him that after many work diversions he has uploaded his audio files to his website. And guess what -- he has a brand new book designed for the standard GCEA ukulele.
I think this is the best book on the market for a baritone
ukulele player with beginning to intermediate skills.
Bruce fills the book with theory, technique and wit.
Here's the link to his music book website, Skeptical Guitarist.
Here's the link to his Baritone Ukulele from Scratch book.
Note that his sound files are downloadable.
Another book, I recommend is Lil' Rev's Hal Leonard
Baritone Ukulele Method Book 1. These are the two best
baritone ukulele method books on the market today.