"Augustus Rookwood ...convicted of leaking Ministry of Magic secrets to He Who Must Not Be Named"
-- caption in the Daily Prophet after he broke out of Azkaban (OP25)
He worked for the Department of Mysteries but had agents throughout the Ministry, not all of whom knew who they were really working for. (For example, Rookwood was an old friend of Ludo Bagman’s father, and Bagman passed him information, apparently quite innocently.) Karkaroff named Rookwood during his own trial (GF30), and Rookwood was sent to Azkaban but escaped in the mass breakout of January 1996 (OP25). Rookwood fought in the Battle of the Department of Mysteries (OP35). He is described as being pockmarked and stooped.
Harry had a vision through Voldemort’s eyes (via Horcrux) of Rookwood kneeling and explaining why Broderick Bode would not have been able to remove a prophecy ball from the Department of Mysteries because it did not apply to him. Due to the information given by Rookwood, another Death Eater named Avery was punished by the Dark Lord, which made Harry’s scar burn (OP26).
Gathered information for Voldemort when he worked in the Department of Mysteries.
rook = "European crow," from Old English hroc, Middle Swedish roka, Old High German hruoh "crow," 1570s to mean "a cheat," especially at cards or dice.
Augustus Rookwood was originally called "Algernon" in the first British Bloomsbury editions of Goblet of Fire, but that was changed in subsequent editions.
One of the meanings of "rook" is to con, fool or trick - apt, for a spy. "Rookwood" also suggests a rather sinister passage from Macbeth (an appropriate play to provide any wizard's name, particularly a Dark wizard), act III, scene 2 -
Light thickens, and the crow
makes wing to the rooky wood.
Good things of day begin to droop and drowse,
whiles night's black agents to their preys do rouse.
In British history, a man named Ambrose Rookwood was involved in a plot to blow up the House of Lords, and for his role in what came to be known as the "Gunpowder Plot," Rookwood was locked in the Tower of London and later hanged.
A rook is large noisy blackbird related to crows and ravens. It can also be a chess piece shaped like a tower, called a castle.