If you, for arguments' sake, pick Roy Jones, you would have to go and spend time interviewing him at his home for the book to have any gravitas. You would also have to research his opponents and speak to them too so that you get a rounded context. Expensive, time consuming, and hard work...
Would be very, very satisfying for you though when it was published.
And if the fighter you did the book on is alive, no doubt he would become a great friend.