You can find the answer to this in the second letter in the book.  It is dated 28th March and is written from Arkangel.

In this letter Walton is writing to his sister Margaret.  He is talking, in part, about regrets he has about his past and the way his past affects him now.  This is the part where he tells what he read during those 14 years.

He says that he was completely self educated and all he ever read was his Uncle Thomas' "book of voyages."

This distresses him because he is now 28 but is less educated than kids who are half his age but who have had proper educations.