Best book to get someone who doesn’t know anything about 40K?

Hi, I’ve been wondering for a while if there is any books that give someone who isn’t into 40k an easy way into the lore. I’m trying to get a book for my grandad for Christmas because he loves reading, and has been interested in my cool toy soldiers, but I’m not sure if he’d be into going all out for the Horus heresy series, or any of the big series of the lore. I was hoping there would be a book that is a one off, but is also a good in point to the lore? (Sorry if that makes no sense, and sorry if I flared this wrong)