A cool thing to note is that you can't read Kindle "Textbooks" on any first-party Amazon e-ink device but you _can_ read them on a Boox e-ink device precisely because it runs Android (read: eTextbooks are supported in the Kindle Android App but not native Kindle devices).
Not to mention you can read other textbook-like sources on e-ink like O'Reilly Books and Scribd as well through their Android apps.
Source: Bought a Boox Max Lumi and it's been the best device I've purchased in years.
You can turn on paged reading rather than scrolling (and fiddle with some of the other settings around color conversion which had bad defaults) and it's as good as any other app.
I read an endless stream of academic papers, which why I opted for the lumi. Most textbooks are less dense (in my experience), so would probably be fine on an Air.
Not to mention you can read other textbook-like sources on e-ink like O'Reilly Books and Scribd as well through their Android apps.
Source: Bought a Boox Max Lumi and it's been the best device I've purchased in years.