"The Mother-In-Law Diaries" by Carol Dawson

From Google Books:

Much to the surprise of his mother, Lulu Penfield's first-born son has gotten married. It's not that she minds; she's fairly sure that he's made a good choice. What's really eating at her is the realization that she is about to become--gasp--a mother-in-law. Lulu's been around the block more than a few times. She's had more than the average number of mothers-in-law, and she's found herself more than once figuring out how these women manage to embed themselves in her marriages. Now there's Lulu, about to become the sacred mother to whom the daughter-in-law must bow, about to become the source of wisdom about the son--his sins, his past, his potential.

Written like a letter to her son. Story sort of jumps around to the different relationships Lulu has had, but overall an enjoyable book!