[erganemic]

I can second the comments that a solid E ink device makes reading pdf/epub stuff a lot more enjoyable, and I greatly prefer it to reading on a traditional screen.

That said, for exactly the class of books (didactic/reference) that your question calls out, there's no substitute for physical books. For me, so much of reading a book meant to instruct involves flipping back quickly to an earlier chapter to refresh my memory on the exact definition of a concept, or paging rapidly through a section to see what headings it covers, or switching from a page in one part of the book to a page in an entirely different part so I can compare their content. There just isn't a frictionless analogue for this, even with E ink devices: and when you're trying to learn stuff, a solution with friction is barely a solution at all.