Brilliant

Respected sociology professor Diane Cole has written the book on love.
Every matter of the heart is neatly explained, and Diane's life spins
in a smooth circle. At her fortieth birthday party a beautiful young
stranger named Ronnie Frost surprises her with a kiss, and the
predictable story that is Diane's life is complicated further when
Ronnie appears in one of her classes.

After Diane agrees to accept Ronnie as her teaching assistant, Diane
finds her views on love challenged by her own heart, as she fights the
attraction she feels for a woman half her age, a woman who is brilliant
in every way.