He scored 33 in 45 appearances in all competitions, including 20 in 31 in the league and eight in eight in the Champions League, further enhancing his reputation as one of Europe's elite strikers.
Brook played 453 games for City between 1928 and 1940 and led the club's scoring charts since overtaking Tom Johnson's total of 166.
City's fourth victory out of four in Group F confirmed their progress with 12 points.
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Seemingly relieved by the Serie A table-toppers halting an assault that lasted for most of the first half hour, City centre-backs Nicolas Otamendi and John Stones converted set-piece routines either side of half-time to turn the game around.
The opening goal had been on the cards for some time when it arrived, with Mertens' delightful flicked return pass finding a finish to match from Insigne.
A goal away to Bradford City in September 1938 moved Brook - capped 18 times by England - past Tommy Johnson's previous best mark of 166, but Aguero has surged to the top of the standings in double quick time. Fernandinho will look to protect the backline, with Ilkay Gundogan and Kevin De Bruyne adding creativity.
However, it only took the visitors less than 15 minutes to pull back an equaliser and it now stand 1-1 in Naples.