That is the approach that is used, though it gets tricky. The items that an inmate can own have to be clear (so they can't be used to store contraband) and tamper proof... and if they're electric, run on regular batteries or wall current.

This leads to things like a clear typewriter - http://www.swintec.com/clear-typewriters/10-2410cc.html

The tablet that meets that requirement is made by JPay - https://www.vice.com/en/article/pgav3m/a-clear-plastic-table... and it does have ebooks - https://www.jpay.com/education.aspx . I will point to their wikipedia page as something to read about them - https://en.wikipedia.org/wiki/JPay

One of the considerations for this is that no-one is going to give wifi and unrestricted (and unmonitored) access to the internet at large. All of the "this is stuff that you can download" needs to be accessible via another set of servers hosted somewhere with all of the associated infrastructure.

Seems dangerous. The external server might base64 some fentanyl and send it along as a header.