Melinda Bilyeu, Hector Cook, Andrew Mon Hughes, The Bee Gees: Tales of the Brothers Gibb (2000)
This is nice biography written by nice people about a group of nice guys. If you are a fan, you will love it. If you're looking for an analytically sharp look at the Bee Gees' contributions to music history, that book has not yet been written.
Alice Echols, Hot Stuff: Disco and the Remaking of American Culture (2010)
If you are interested more in disco from an historical and sociological point of view, read this book from American Studies professor and Janis Joplin biographer Alice Echols.