Books I recommend for writing BDSM:
Screw the Roses, Send me the Thorns
Miller & Devon
SM101
Jay Wiseman
The Topping Book
The Bottoming Book
Easton & Liszt
The Sexually Dominant Woman, a workbook for nervous beginners
Lady Green
The Fine Art of Erotic Talk
Bonnie Gabriel
Men are from Mars, Women are from Venus
John Grey, PHD.
For writing BDSM, I can't recommend these books highly enough.