That's big enough for a Kiwix English Wikipedia snapshot, so I'd start with that. The Project Gutenberg epubs could keep you busy for a lifetime, so throw those on too. Some US Army field manuals might be good for basic survival instruction, though these are things you should practice before you're actually in a survival situation.

In this apocalypse scenario, I'm probably more worried about finding potable water and fending off scavengers than what's on my tablet though. It's a fun thought exercise, but not terribly practical. Buy paper books if you want something offline.