I started of playing the standard GCEA tuned painted Harmony ukulele that my father bought in the 1960's and gave to my kids where it was ignored for about twenty years. After a few years I found that I liked the low G tuning but when I bought a vintage baritone ukulele I completely switched over to the DGBE tuning.
While some standard GCEA tuned ukulele players are picking up the bari-uke and then tuning them GCEA to avoid switching, this blog almost completely stays with the traditional DoGBonE tuning. It is more traditionally called "Chicago" tuning and is the same tuning as the four high (pitched) strings of the standard tuned guitar.
The four books recommended below all have DGBE tuning. You can see the full list of published baritone ukulele books since 1950 by clicking on the B=O=O=K=S page above.
The first book I recommend is Bruce Emery's Baritone Ukulele from Scratch. Bruce played the baritone in his teens and as an adult he has a near cult following of his self-published Skeptical Guitar Books. Bruce's wit and knowledge make the book very readable. He also goes over several chord families for the most frequently used keys. It is packed with a lot of information that will keep you exploring from beginner to intermediate player. (Tracks may be heard via an online site.)
The second book is written by a friend of mine, Lil Rev. and is called Hal Leonard Baritone Ukulele Method. He's a multi-instrumentalist, storyteller, teacher and writer. He's a fan of old-time music, blues, double-entendres. This book is published by Hal Leonard and they always make high-quality publications. (The book comes with a CD of all of the examples.) It is a great book and all of the material is achievable.
The next book, Baritone Ukulele - Fretboard Roadmaps, will be released May 1, 2017. It is written by another prolific musician and writer named Fred Sokolov. I would not purchase this as my first book but as a secondary book as ones skills are progressing. Fred shares a lifetime of experience and is the wise uncle showing all of the short-cuts. Not that this is easy stuff. But it is the wisdom you will want to have as you continue. If you're new to string instruments this one can wait a few months.
Ukulele Aerobics by Chad Johnson is another book stuffed full of Baritone Ukulele "workouts." There are 40 weeks of 7 lessons per week. Several musical types and techniques to build increased speed, improved dexterity,better accuracy, better coordination, as well as increased melodic and harmonic vocabulary.
The books are all priced at $17 or less and you will surely learn a great deal from any one of them.