These discoveries are old enough to be in the textbooks already.

Not sure what would be good popular science books. There is quite a lot on the immune system in the Alberts (Molecular Biology of the Cell), but that is maybe too much without solid biology background knowledge. The typical textbook is the Janeway (Immunology), but that's certainly too much.

What I liked as an introductory textbook in general was Campbell Biology, but that covers essentially all of Biology. There is a chapter on the immune system as well.

All those books are horribly expensive in the US, and still quite expensive in other countries, though.