Not mentioned much in the game books, but here is a quick chart for reading and writing in my campaign.

intelligence     reading level                             writing level
   10              basic skills                            basic skills
   11              can read books with simple concepts    can write fairly well
   12              can read basic scholarly works         can write basic scholarly works
   13              can read most scholarly works          can write most scholarly works
   14              can read all scholarly works           can write all scholarly works