Immoral / Forbidden by Jason Hytes; Kimberly Kemp

The blurb on the back:

Two kinds of love treated separately and brilliantly by two great authors...
Immoral - This gripping novel proposes the following question: Can love between husband and wife not only exist concurrent with physical infidelity but be, in fact, nourished and strengthened by it? True or false?
Forbidden - It began as a curious experiment, strictly a one-time-thing, a harmless excursion into a strange and fascinating twilight world. But one having tasted, Holly Kingdon discovered she could not do without more of the forbidden fruit.

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opening lines:
The town of Demeter was gripped by a merciless heat wave, a fact made official by the afternoon newspaper which announced to one and all that it was the hottest June tenth on record and that there had been several heat prostration cases in town as well as a drowning at the lake.