That site links to the 2005 edition of the book The Elements of Computing Systems, but an updated edition was published in 2021. (I found the original edition excellent years ago. I'm not aware how the new edition differs, only that it exists.)

https://mitpress.mit.edu/books/elements-computing-systems-se...

https://nandgame.com/ is also great

edit: oh, and there's new levels. there goes my afternoon...