Book suggestions for Alzheimer's patient?

My dad was recently diagnosed with Alzheimers. He still likes/tries to read, but he can’t follow a full length book, so I thought I might be able to find a collection of short stories or articles that he might enjoy. In non-fiction, he’s interested in Mars, space travel, and geology. In fiction, he used to read all of the Jack Reacher books and things of that ilk.

Do you have any recommendations?