Underboss is very good, and you come away with a reasonable assumption that Gravano is telling the truth about most things.

If you want a good book about Gallo, read "Joey" by Donald Goddard (straight-up biography) or "The Sixth Family" by Pete "The Greek" Diapoulos (written by a former associate of the Gallo crew, it mainly focuses on the time period between Joey's release from prison and his death).