Player's Handbook[]
The character can read and write a modern language he can speak, provided there is someone available to teach the character (another PC, a hireling, or an NPC). This proficiency does not enable the character to learn ancient languages (see Languages, Ancient).
Player's Option: Skills & Powers[]
Reading/Writing: The character is literate in a language that is contemporary to the campaign world, provided that the character can speak it (see the modern languages proficiency), For each additional character point spent on reading/writing, the character is literate in one additional language.
The Complete Thief's Handbook[]
Recommended: Investigator, Spy.
This proficiency is normally restricted to priests and wizards. Its description is on p. 61 of the Player's Handbook.
Campaign Option: Council of Wyrms Setting[]
The Reading/Writing proficiency is described in the Player's Handbook. It allows a dragon to read and write a modern language it can speak. Each of the dragon languages has a written counterpart. Unless a dragon has a Dexterity score of 13 or better, it cannot make use of pens or brushes for writing on scrolls. Without a high Dexterity, the best a dragon can do is carve runes with its claws.
Hatchlings can't take this proficiency.