From Enterprise News:

On the heels of winning the title last June, Rivers has the Celtics off to a 17-2 start and he was named on Monday as the Eastern Conference coach of the month for November.

He also won the award last March and April and becomes only the third coach to win the honor three months in a row since it was established in 1981. Flip Saunders (2005-06 with the Detroit Pistons) and Larry Brown (2002-03 with the Philadelphia 76ers) also achieved that feat.