Alyssa's Needs Trilogy, Book Three by Olivia M. Ravensworth
Volume III reveals the final reward of the mistress's groveling worshiper. First, though, of course, Brad will serve his mercilessly commanding angel-whore in ways he never imagined a man could, no matter how committed a husband and devoted a lover he might be.
In addition to using her own body, after all, a highly sexual woman of inventive and uninhibited imagination might find other strange and unnatural ways to amuse herself, ways which neither her many, many boyfriends nor her shocked husband would prefer. No one, however, can resist the lovely Alyssa. Eventually, therefore, submit Brad does, squirming in embarrassment and yet thrilling helplessly to the look in his goddess's approving dark eyes as he swallows more than just his pride.
One dark midnight, though, the remorselessly cuckolded husband is wakened once more by the sounds of lovemaking in the bed right beside him. But rather than the anonymous man whom the husband naturally had been expecting, Brad blinks his eyes wide to see a fleshy middle-aged woman and a skinny young girl with long red hair and delicate tiny breasts cavorting with his unpredictable wife. The confused Brad has never imagined the like, but of course only Alyssa knows the true fate that awaits her sweetly degraded devotee...