I was never that into Manson, but picked this up at the store on a whim and found it pretty captivating. Doesn't waste time mythologizing him (nor does O'Neill make any concrete hypotheses), but goes into some incredibly well-researched detail regarding MKUltra and the CIA's interest in using Haight-Ashbury at the peak of hippie culture to perform "field research" in testing the possibility of mind control with combinations of LSD and amphetamines.
There's a lot of irrefutable evidence that Manson was on the radar of the FBI / CIA for a while before the Tate-LaBianca murders and that they seemed to have a vested interest in keeping him on the street despite innumerable parole violations that should have landed him behind bars.
Highly recommended.