I read this book too, and... it's a mess in terms of chronologies, names, etc. just basics of biography. It's very often...frustrating and discouraging.
In spite of that, there are some good insights from Rollins family members, as well as a few things that I can't say have been covered elsewhere, like some kind of feud between Sonny and Max, and Sonny's fondness for levitation.
There's also a focus on Sonny's spirituality as indicated by the title. The author appears to be personally oriented toward that way himself and provides background on all the different paths that Sonny travelled.
Bottom line - not at all "essential", barely "supplemental", yet still - not without bits of interest sprinkled in all the way through.