I agree that there aren’t that many. I just started looking through my list of things I’ve read in the past several years (in an app on my phone), and I could find a fair number of non-fiction books, but very little fiction. “Gentleman Jole and the Red Queen” by Lois McMaster Bujold came to mind, but you have to have read the whole Vorkosigan series to appreciate it, as it is at the end (although “Cordelia’s Honor” is about a reasonably mature woman, I think late 30s, and it is the first in the series).
Thanks all. I’ve read many of these, but there are lots for me to look for in the library.
I am floored. I just finished going through the list of 479 books I’ve read in the past 8 years ago. About half are fiction. Of those, except for the Bujold ones mentioned above, I found ONE fiction book with a middle aged or older woman as protagonist. That was Rollback by Robert J Sawyer. So… 3 out of about 240. 
The Shell Seekers by Pilcher is about a woman with grown children.
Ditto @intparent post #22: I’ve read 465 books but found few with middle-aged or older women protagonists. Most have been mentioned and the others not good enough to recommend.
Hmm. Of the 366 I’ve read in 6 or 7 years, 36 have middle-aged or older women as the protagonist or one of the main characters.
Almost anything by Penelope Lively. She’s one of my favorite authors. British.
Mrs Emily Pollifax series by Dorothy Gilman—loved those books. She’s a woman with grown children who goes on many exotic adventures for the CIA. Our library had them.
Ditto the Anne Tyler recco
Also check out books by Meg Wolitzer - she’s an excellent writer and story-teller as is Julia Glass
“I think of middle aged as anywhere from about 45 to 65.”
As my mom told my 56 year old sibling who referred to himself as middle-aged, “You’re not going to live until 112”. Things moms can say. 
I recommend books by Ann Pratchett or Jane Smiley.
Also dittoing Ann Tyler and Penelope Lively. Everything by Margaret Drabble in the past 30 years or so. I wish I had a better “books I’ve read” list because I feel like I’ve read tons of them. Middle-aged middle class women are the main reading public, I’ve heard, and I lots of books are written with them in mind, by middle aged women.
it’s just my middle-aged brain not remembering them at the moment.
Oh, lots of Elinor Lipman’s books. Meg Wolitzer. Barbara Pym. Hmmm. I know there’s others I’ve read. I’ll come back here when I remember them!
Diana Gabaldon’s Outlander series after you get past book 2.
“Summer Before the Dark” Doris Lessing. It’s an older book, but holds up very well imho. Many of her books have middle aged female protagonists.
Margaret Atwood is one of my favorites for more recent stories. “Stone Matress” is all stories about women of a certain age, if I remember correctly, and I especially enjoyed the story about a woman in a fancy retirement community, as she lives through the end of civilization as we know it.
adding: It’s “Our Turn”
Lessing and Atwood both write older and older women as they age. I like that a lot.
adding:
Kate Fansler Mysteries, by Carolyn Heilbrun. Kate is an elegant professor solving academic mysteries. She has no children but adult nieces and nephews appear.
Amelia Peabody Mysteries, by Elizabeth George. Amelia is a late 19th/early 20thc archaeologist. The books follow her Indiana Jones style life. While she digs in Egypt, she solves mysteries and raises a family. The children grow up and marry and have children of their own. There are 20 books so they will keep you busy a while if you care for them.
Amelia Peabody author is Elizabeth Peters, not George.
Thank you, garland.
Dumb question: how are people keeping track of the books they have read?
I use Goodreads to catalog books I want to read and books I have read.
I use Goodreads, too.
Another Goodreads user. My only connections on Goodreads are my 2 kids and their spouses. I enjoy seeing what they are reading and often get ideas from them for my future reading.