If you go for free and online anyways, then you can pursue more interesting formats like Quantum Country [0]. It seems to include a spaced repetition mechanism. I have yet to try it though.
Love the idea of a book based on spaces repetition, but wish they had a better way to get a sense of what you’re getting yourself into, since as far as I am able to tell there’s no:
- estimate of reading time
- level of knowledge
- way to link deeper content
- estimate of reading time - level of knowledge - way to link deeper content