Linux

That was just the first example to pop to mind where you couldn't just grep search * and I didn't want to get into a bunch of specific file formats. For something like epub you could probably just use zcat and then pipe the output to grep. Perhaps using a for loop if you want to do other fancy stuff along the way (e.g. output file names as headers).

So ya, "hard" may have been a bit overblown. "not simple" may have been better. But, without the OP actually stating what format the ebooks were in, I wasn't going to write a primer on dealing with any format.