| Quote #4
"That's Orion down yonder; the big fellow to the right is Aldebaran, and the bunch of little ones-like bees swarming-they're the Pleiades..." (1.12)
Ethan obviously has an intimate knowledge of the night sky, and delights in showing this to Mattie.
| Quote #5
The fact that admiration for his learning mingled with Mattie's wonder at what he taught was not the least part of his pleasure. (1.12)
Ethan would also have made an excellent teacher, it sounds like, though we can only assume that Mattie is learning from as he says.
| Quote #6
When she tried to extend the field of her activities in the direction of stenography and book-keeping her health broke down, and six months on her feet behind the counter of a department store did not tend to restore it. (3.6)
From this we gather that Mattie isn't used to studying. We know she can read, because she reads Ethan's letter, but whatever her education is, it must be sparse. As a result, she is extremely limited in her options.