This is the best baritone ukulele book made for beginning to solid intermediate player. Bruce Emery has written several guitar books in his Skeptical Guitarist series and he takes this experience and makes a first class spiral bound book. I recommend this book very highly.
Next we have my friend Lil' Rev's Baritone Ukulele Method Book 1. I am very partial towards Lil' Rev since I have taken several of his classes and seen him in concert several times. Rev is a multi-instrumentalist, historian, and story teller. This book is also in my collection and it is a basic Method Book style full of Rev's style. (I am also honored to have this blog, "Humble Baritonics" mentioned in the book.)
Now some kind but anonymous reader left mention of this book in the "Today is the Best Day to..." posting comments. I have not been able to peak inside yet but I do try to purchase all books that support the Big-Uke. This book is for a standard uke in C or D tuning (GCEA or ADF#B) and G tuning (DGBE). I will keep my eyes open looking for these at Gryphon Strings in Palo Alto, CA.
Here's another book mentioned by Anonymous that I currently know nothing about. Several people have posted Gospel Tunes on YouTube. The book specifically mentions "Baritone Ukulele." Found an Amazon link to Ondrej's books:
This book is of famous tunes compiled by Jim Beloff. It is a standard for every Bari-uke players collection.