All of the original ones by Asimov, several times over. Including books like The Stars, Like Dust that can be loosely included in the Foundation saga and pretty much every short story. I must admit I set myself for disappointment, but they have only kept maybe one paragraph of the original premise, and some characters' names.
Butchered for tellie adaption is probably what you feel like when watching the adoption. I feel the same about the Hobbit movies or the current Wheel Of Time show. In return, having not read the Foundation books, I quite enjoyed the show - likely based on my missing book knowledge.
It went from political intrigue to an action thriller with every cliche they could throw at it: countdowns to destruction, bombs, bows and arrows (for some reason), killer robots (they apparently decided to just ignore Asimov's own three laws of robotics)...