my fave series is this one: The "Joe Gray" series by Shirley Rosseau Murphy. This new series featuring Joe Gray, a tomcat, and Dulcie, a library cat in the Bay Area town of Molina Point, led to some wary hissing in the world of kitty lit. That's because Murphy's cats are not only fully conversant in English, but literate as well. This violates what had been the literary cats' prime directive against inter-species communications. However, it has led to a lively series, opening with Cat on the Edge (1996), and now up to a fifth installment, Cat to the Dogs (2000). I confess I've approached these books with real caution. Of course, I have no trouble believing that cats talk to one another, but I have difficulty understanding why they would stoop so low as to speak to mere humans.
she also wrote a fantasy book, The Catswold Portal, about a breed of humans who can transform into cats - i so wanta be one! it's good, too, just different - not a mystery, per se, although there are some mysteries within the story.