"Underboss" was a good read, despite being hypocritical. Gravano ridicules Castellano for being a mere "racketeer," while he and Gotti were honest-to-goodness "gangsters." Then he reprints every complementary word Castellano ever uttered about him. Then he denounces Gotti. Killed 19 people, including close friends and relatives, because he's a real "mafioso." Yeah, a real SS storm trooper ("I vas ohnly obeying orders"). Jerk.
Plenty of books about Gotti, including "Goombata," which isn't bad. Organized crime books generally can be good page-turners but are seldom authoritative or deeply researched because gansters generally aren't the types to write autobiographies (except sanitized ones, like Bonanno's, which is still a good read) or leave their collected papers to some university for researchers to peruse. Two excellently research books I recommend: "Little Man," the Meyer Lansky biography by Robert Lacey, and "The Black Book and the Mob" by Farrell and Case, which is the best book extant on Nevada and gangsters.