It’s a unique premise for a memoir. Novelist A.M. Homes was the product of an affair.

Her much-older married father began a five-year relationship with a 17-year-old clerk at one of his stores. He said he wanted to marry the mistress. He said he wanted them to move to Florida. But when his mistress and his wife end up pregnant at the same time, everything changes – at least for the much older man.

Meanwhile, Homes’ adoptive parents had just lost a 9-year-old son to kidney disease. A hysterectomy had left the woman unable to have more children, so the couple set up a private adoption through their lawyer who announced on Dec. 18, 1961, that their “package had arrived, it’s wrapped in pink ribbons and it has 10 fingers and 10 toes.”

Homes’ birth mother, 22 and unmarried, felt she had no choice but to give up her daughter.

“Yes, I have always loved this little girl and been tortured every December of my life from the day she was born that I did not have her with me,” she writes to her daughter, the author.

Homes holds back little in the first half of “The Mistress’s Daughter” (Viking, $24.95). Her pain, confusion and sadness are palpable and it makes for an extraordinarily compelling read. But something falls apart in the second half. She begins “Book Two” with a promising chapter called “Unpacking My Mother.” After her birth mother’s sudden death, she packs up some of her belongings, then refuses to look at them for seven years.

Homes devotes too much time to discussing her genealogical research and an imaginary deposition in which she asks her birth father everything she’s always wanted to know. A tacked-on chapter about her adoptive grandmother seems an afterthought.

The disjointed and incomplete feel of the book’s second half makes one wonder whether Homes, whose book was released just two years after unpacking her mother’s boxes, hasn’t had quite enough time to take it all in.