A couple I recommend are:

Carrie's Story by Molly Weatherfield
Safe Word by Molly Weatherfield (sequel to Carrie's Story - almost as good as the first)
Gordon by Edith Templeton (if I remember correctly, it wasn't as sexually explicit, but it delved more into the psychology of BDSM)
King's Pawn - I can't remember the author's name, but it was published by Black Lace.