This was a hard choice because a book doesn't have to be funny to cheer me up, sometimes the memories associated with it are enough.
There is one memorable scene from Evanovich's Stephanie Plum series where Porsche meets garbage truck that never fails to make me laugh. I love the banter between J.D. Robb's Eve Dallas and Roarke and Eve's give and take with Peabody.
I chose Die For Love for it's heroine, Jacqueline Kirby, and her hat (if you've read the book you know what I mean) and the fascinating contents of her purse. Kirby has been a favorite since the moment I encountered her flat on the floor of an Italian university, The Seventh Sinner. A strong, sexy, snarky woman who gets her man and the murderer, how can that not cheer you up?
I also chose Crocodile on the Sandbank because it was the first of the adventures of Peabody and Emerson. Amelia Peabody and her parasol set out to see something of the world and nothing but Emerson and a "mummy" got in her way. "Every year another body."- Abdullah.