Published by G.P. Putnam's Sons, "The Cat Who" book series contains 29 mystery novels written by Lilian Jackson Braun in a three-series set. The first book was released in 1966, followed by the second and third books in 1967 and 1968, respectively. It was eighteen years before the fourth book was published, however, after that point, a new book was released every year until 2007. The last book was never published due to Braun's death in 2011.
The main character in the series is Merlin James Qwilleran, or Quill. He is described as a tall man with dark eyes and black and gray hair. His defining physical characteristic is his "luxurious" moustache. He credits his moustache with alerting him to dangerous situations or times his instincts are exactly right. Quill inherited a large fortune, but he lives humbly and donates a lot of money to charitable or civic causes.
The reason the books all start with "The Cat Who" is because of Quill's Siamese cats Kao K'o-Kung, or Koko, and Yum-Yum. Quill's bond with his cats is very strong, and there are several story lines where his cats are affected by his current mystery. Both cats are very intuitive and have a unique way of helping Quill with his cases. Quill loves his cats so much, he risks his life for them in more than one story.