Reading and writing is a skill that not all are blessed to learn; sometimes, even speaking properly can be hard for the uneducated. For Mages, this problem is increased, as many arcane tomes are written in languages not used anywhere else.
At Rank 1,you gain the ability to read and write, and can use the skills wherever appropriate. (Writing books, renaming signs, etc.)
At Rank 2,you begin studying arcane languages, increasing your Skill when making Tomes by 1.
At Rank 3,reading such arcane languages is as simple as reading an Inn sign, increasing your Will by 2 when using a Tome. In addition, you can equip Tomes while using any class.
At Rank 4,you're used to carrying heavy books around, reducing the Battle Weight of all Tomes you use by Rank.
At Rank 5,you are really good at the word thing, increasing your WIL growth by 5%.