I recently (by phlog standards) got myself an e-boot reader -
specifically a Kobo Sage. I am quite enamored with it as a device. It
has an eink display, meaning there is no need for a backlight, but you
can instead rely on ambient light. This makes it much more like
reading a piece of paper. The interface is very simple, comprising
three buttons and some touch capacity.

It is essentially a single-purpose device: it can show eBooks in
various formats, including PDF, and that's it. I was a bit nervous
when it first booted and asked me for a wifi password, but I've since
just ignored that capability. I feed it with files over a standard USB
connection, and then I read them. The battery life is extremely long,
likely due to the passive display, and it is extremely lightweight. I
expect that even in the absence of software updates, this device will
remain usable for at least a decade, assuming the battery doesn't
suffer some trauma.

I mostly use it for reading ebooks in the epub format or equivalent.
It is also tolerable for reading academic papers in PDF, but only if
they are single-column.