It's been a long time since I read it but if I recall correctly the book posited that Giancana was personally involved in or had ordered virtually every mob hit since the St. Valentines Day Massacre as well as the Kennedy assassinations and the death of Monroe.

Even for an occasionally credulous person such as myself that's a little hard to believe. While I don't doubt that loved ones get some inside info that outsiders don't the book almost depicted Giancana as an Iago like villain who just loved telling his little brother all the evil stuff he was doing, had done and was planning to do.

So I think like a lot of other books it's a mix of fact, conjecture and pure fiction.