All three foundation books. I read them before watching the series and they were incredibly good. Although you can watch the series first if you want, the TV series and the books cross over about as much as the Harry Potter books and LOTR do, there’s wizards and baddies, that’s about it
I reread the first one and was struck by how much smoking was included. (H. Beam Piper too) Between that and the nucleics, it really felt like 50's era SF, admittedly one that held up better than the bulk of them.
