Best novels about masturbation?

One shortcoming of the LoveHoney site is that you can't search the books for ones about a particular subject. Personally I particularly enjoy stories that feature masturbation, but there is no way of tracking these down. Does anyone have any suggestions about good books which feature a good dollop of masturbation as part of the story, preferably mutual or group?

I can put two candidates forward that I have much enjoyed and revisit often:

Forbidden Fruit by Susie Raymond - a great book - very well written and with lots of action.

In Too Deep by Portia Da Costa

Any other suggestions?