Ottoline and the Yellow Cat by Chris Riddell

From chapters.ca:
An exciting new character- based mini series for young readers. Introducing Miss Ottoline Brown, an exceptionally inquisitive Mistress of Disguise, and her partner in crime, Mr Munroe. No puzzle is ever too tricky for the two of them to solve ... Ottoline lives in a stylish apartment in Big City with a small hairy creature called Mr Munroe. Together they look after the Brown family's eclectic collections - and dabble in a spot of detective work. So they are the first to the scene of the crime when a string of high society dog- nappings and jewel thefts hits Big City. Ottoline (who luckily has a diploma from the Who-R-U Academy of Disguise) and Mr. Munroe go undercover - and expose an ingenious scam masterminded by furry feline crook, the Yellow Cat.