I recently build myself an ebook reader from a Nexus 7 using Ubuntu Touch:

https://www.exratione.com/2015/10/turning-a-nexus-7-fhd-tabl...

Dealing with other tablets and Touch is pretty hit and miss - you might be able to get it to work, but more likely not unless you're pretty experienced in low-level hacking in OS and devices.

Certainly that hardware doesn't have enough memory to keep Beru happy all the time, which is a pain, but I'm prepared to put up with the minor inconvenience in exchange for not having an reader that reports everything I do back to central.

If an official Ubuntu tablet can be disconnected from their report-everything-to-central mechanisms to the same degree that Ubuntu on the desktop can, then an official tablet may be worth the additional cost just to use as a reader.