The Midnight Louie mystery series by Carole Nelson Douglas. Our favorite series! Midnight Louie is a big black tomcat who thinks he's a private eye in the tradition of Sam Spade and Mike Hammer. He prowls the neon casinos, back alleys, and desert sands of Las Vegas, and solves mysteries for his "little doll," crack PR person Temple Barr who is torn between her love for the athletic magician and counter-terrorist Max Kinsella and the former priest-slash-radio talk show host Matt Devine. There are almost as many animal characters as humans, and Ms. Douglas has weaved an intriguing story arc into the individual books' plots.
While the self-contained mysteries often get shuffled into the background in favor of the romantic and personal entanglements, and the wordplay can be a bit much at times, Staffpurrson devours each one of these books as soon as they come out. There have been 22 books so far, the latest of which is "Cat in a Topaz Tango," and word has it that Ms. Douglas will solve the Big Mystery in the 28th book -- if she sticks to the alphabetical titles all the way through U, V, X, Y, and Z. (Still trying to figure out what color she'll find that begins with the letter "X".)
The Eve Dallas mystery series by J.D. Robb (aka Nora Roberts). While tough NYPD detective Eve Dallas and her filthy rich and incredibly talented husband Roarke (yes, just Roarke) are supposedly the stars of this series, my favorite character is their fat gray cat, Galahad. He made his presence known in the very first book, Naked in Death, by saving Lieut. Dallas from the murderer, and shows up periodically in the 30-plus books published since then. He was amply rewarded for his heroism with a lush life in Roarke's palatial mansion, cadging bacon from their plates, sleeping on top of Eve's naked bum, and getting along better than Eve does with Roarke's steely-eyed majordomo, Summerset. He may not show up as often as Midnight Louie -- after all, it's the Eve Dallas series, not the Galahad series -- but he's always welcome in my house.
Staffpurrson also used to read The Cat Who series by Lillian Jackson Braun, featuring KoKo and YumYum, and the Mrs. Murphy series by Rita Mae Brown, but he feels those have fallen off in quality in recent books. The early ones are still good reads, though. There's also Jad-bal-ja, the Golden Lion, who was featured in Tarzan and the Golden Lion by Edgar Rice Burroughs and shows up in some of that series' subsequent books. A most noble fellah is he.