I like Ann Ball’s books about Bl. Miguel Pro . The one for adults is called “Blessed Miguel Pro: 20th Century Mexican Martyr” and the one for young people was called “God’s Secret Agent: The Life of Miguel Pro” but that one is out of print, though you might still be able to find an old copy (it had a great ending!)
Also, while I know it’s partly fictionalized, I really liked “The Song of Bernadette” by Franz Werfel. It gave me a much better understanding of the attitudes in France at the time of the Lourdes apparition, it’s really well written, and the story of how it came to be written is as good as the novel itself.
“Fatima in Lucia’s Own Words” by Sister Lucia is also really interesting as she gives a lot of details about Sts. Francisco and Jacinta Marto from the perspective of her childhood memories. You get to see very clearly how all three of them were really just children and often acted like children - probably better behaved and more responsible than children that age would be today, but still children as it talks about games they liked to play and the way they express themselves is totally childlike.
