

Listed on eBay -- some of Agatha Christie's family's book collection
Dating from the 1880s (several years before Agatha was born) -- if the set was bought new around the time of publication, I guess the original owner would have been Margaret Miller, sister of Mary Ann West. Mary Ann was the mother of Agatha and Madge's mother Clara. It's a bit complicated because Mary Ann was widowed and having difficulties, so Margaret basically adopted her niece Clara and raised her. So she was foster-mom to Clara.
https://agathachristie.fandom.com/wiki/Margaret_Miller
At the same time, Margaret was the second wife of Nathaniel Miller, and stepmom to his son Frederick. When Frederick decided to marry Clara, Margaret became step-grandma to their kids, through Frederick -- while being maternal great-aunt and adoptive grandma through Clara. (Mary Ann West was still in the picture, as "Grannie B", with the B standing for her married name Boehmer.)
Margaret was called "Auntie-Granny" in Agatha's autobiography. Some of Agatha's biographers suggest that she was a major inspiration for Miss Marple.
Someone posted the family tree, showing Margaret's ties to the rest of the extended family.
https://www.reddit.com/r/agathachristie/comments/xjbjtj/it_is_widely_believed_thanks_to_agatha_herself/
(the other relationships don't show up in some of the other family trees online)
https://www.wikitree.com/wiki/Miller-20109
Margaret's signed her name in one of the books. I imagine she gave the books to Madge, who may have taken them up north when she got married. In any case, they eventually ended up with Agatha and were stored at Greenway, as the listing describes.
I suspect that they were auctioned off some time after the property was donated to the National Trust, after Agatha's daughter Rosalind died. People went through all the stuff, kept the personal papers and the most historically-relevant items, and the rest of the things were sold off. (Storage space was limited -- Agatha herself described what a burden it is, to have a bunch of stuff lying around that isn't really important. She had to go through all her family's things at their home in Torquay and clear out rooms.)
So if anyone likes historical books -- in French -- and is interested in things that Christie had at her holiday home ... I've included the "Buy It Now" link.
https://www.ebay.ca/itm/374326010025