Oh boy, the choice is huge. I can recommend Sharon Penman's "The Sun in Splendour" - it is a historical novel based on the events surrounding Richard and Edward IV - absolutely engrossing.
The first book I ever read on the story was Elizabeth Jenkins' "The Princes in the Tower". She says Richard, in all probability, had a hand in his nephews' demise but that there was no other option open to him if he hoped to survive. It is a very good read.
Thanks for the response
I'm off to Amazon now to track those down!