The best biography by far, IMO, is 'Freddie Mercury The Definitive Biography' by Lesley Ann Jones.
There is so much time and depth gone into this book. She even travels to Africa to visit Freddie's home town of Zanzibar and talk to people who live there that knew him/his legacy.
I highly recommend it if there is a Freddie book out there you might want to pick up and read but unsure which one. http://www.play.com/Books/Books/4-/284028/Freddie-Mercury/Product.html?&engine=froogle_books&keyword=Freddie+Mercury+-+Lesley-Ann+Jones&_$ja=tsid:11518%7Ccc:%7Cprd:284028%7Ccat:Books+%3E+Biography