As mentioned in the link above, Assembly Lines is still in print as a paper book (maybe it's print on demand, but it's a hardcover).

I just noticed that Understanding the Apple IIe also got a re-issue from Call-A.P.P.L.E. in 2024 (print edition from Lulu) - https://www.callapple.org/books-3/understanding-the-apple-ii... .

Sather's book is the most comprehensive I've ever read. He even discusses the split screen vblank trick.

Assembly Lines is the one I came to recommend. Great book to sit down with and work your way through.