I recommend Kiwix
I was looking for an offline wikireader and noticed that a GPLv3 one is not in the Trisquel repo's. As far as I can see it is Free as in Freedom so hopefully it checks out with you guys. The main site is here:
See issue 2284.
Basically we are waiting for upstream to package it and apparently they are having some problems. There is nothing wrong with installing it yourself following the instructions on their website considering it is free software.