Relating back to my last question-Isn't Joe impressed by Pip's writing because he, himself can't write?

Joe is definitely impressed with Pip's accomplishment. And yes, part of the reason would be because Joe himself cannot read or write.

Ah!” added Joe, with a shake of the head that was full of meaning, “and begun at A too, and worked his way to Z. And I know what that is to do, though I can't say I've exactly done it.”