Give me wifi, e-ink display, and the ability to write a regular program that writes unicode chracters to STDOUT (and have that rendered to the display) and I'll give you my money right now. All it needs is a plain-old Linux TTY driver for the e-ink display (so you can set the fonts as you would in Linux). Then all you need for e-reading is a plain text file, the column tool for formatting, and the less pager for paging.
In terms of output more complex than text, there's this program dialog that takes command-line input and renders a UI in a console. That exact same thing, except rendering directly to the e-ink display, would work fine for 90% of e-ink apps. It could handle rendering fonts, styles, images, layouts, buttons, etc. Anyone who can learn basic scripting and use unix command-line tools could then make their own e-ink apps, and run them on the tablet, with absurdly low resource use.