Much Better Than The Movie
I realize this book wasn't the one on which the movie, J. Edgar, was based. This is Richard Hack's book, with a far better and more detailed recounting of the life of a man both complicated and very dangerous.
Without Hoover and his information on people of interest, (legally obtained or otherwise), certain presidents might not have been elected or certain laws might not have been passed.
None of this seemed to have been made clear in the film, despite superior acting by Leonardo DiCaprio and direction by Clint Eastwood.
Read this book if you want inner workings of Washington DC revealed.