Why are there so many books? Each different kalimba, or each different tuning of the same type of kalimba, will have different notes, and hence can play different songs. Each will have different strengths and weaknesses, each will need its own music, and each will have its own books. The exception: books written for the Alto kalimba can also be used for the lower 15 of 17 notes of the Bb Treble kalimba as well as for the upper 15 of 17 notes of the D Treble kalimba which both have tines painted in a way that is compatible to the Alto kalimba.
Almost all of these books utilize Kalimba Tablature, a graphical representation of the kalimba extending up the page with musical notes plopped on the tines that need to be played to accomplish a particular melody or song. Every tablature-based book comes with a CD or a URL for a download page so you can hear how each song goes. Tablature does not tell you how the song goes, but it will show you how to play the song. A few books are numbers-based, and a few have both tablature and staff notation.