Murder Machine is a Mafia "must read" imo.

As for Gaggi, I believe, as someone had previously stated in another DeMeo related thread here, that the authors assumed, given the information they had at the time, that Gaggi was the triggerman, but that subsequent information provided by Casso in his book confirmed that Casso was given the order to kill DeMeo, which, along with the Gemini Twins who he was now in control of, he then planned and executed.