In helping the Warden launder his ill-gotten gains, Andy gained several privileges:
1. He was allowed keep minor contraband (the rock chess pieces that he carved) and to accessorize his cell (the all-important posters of Rita Hayworth and others).
2. He was given additional a "favored" status and additional protection by the guards (before this, nothing would happen when the Sister's raped/beat up Andy, but after he became a "favorite", Hadley crippled Bogs for beating up Andy in the projection room).
3. He was reassigned from his crappy job in the prison laundry to a cushy job as Brooks' assistant in the prison library (the first assistant Brooks ever had in his many years as the prison librarian).
4. He is given permission to write letters to the state legislature asking for funds to expand Shawshank's library, which the Warden Norton mails, free of charge (the library becomes Andy's pet project and, besides his ongoing excavation project, gives him a sense of purpose while behind bars).
And when Andy threatens to discontinue his help for Norton (after Tommy's murder), the warden threatens to wall off the library, burn all of the books in the prison yard, and remove Andy's preferential treatment (including protection from the Sisters).