It's called Dorothy On the Rocks, and the author is Barbara Suter.
From the book description (the book is set for release at the end of June, in paperback):
"In Maggie Barlow's world, reality is overrated. So what if her singing career has hit a sour note or she's no longer the ingénue that she used to be? So what if she drinks and smokes a bit too much or likes to chat with a fairy godperson who appears to her from time to time? She's the queen of denial and an actress to boot—she can just take on the role of someone she likes better than her sorry self. Regrettably, that role is currently Dorothy in the Little Britches Theater Company's production of The Wizard of Oz.
Dorothy on the Rocks is the story of a funny, lovable, totally self-destructive woman who, after a night of one-drink-too-many, wakes up with a strange man in her bed: confident, handsome, sexy, twenty-eight-year-old Jack. What happens next is what makes Barbara Suter's coming-of-middle-age tale so much fun. For when the make-believe is finally stripped away, our hurt, lonely, and very afraid heroine finally takes center stage and finds herself starring in a totally improbable love story. It just might be the role of a lifetime."
Yikes! Sound like Laurie Notaro (she's most famous for her memoirs, but her novel There's A Slight Chance I Might Be Going to Hell is a hoot) meets Deanna Kizis (How to Meet Cute Boys) meets Jen Lancaster (Bitter is the New Black). What a fun summer read this could be. And the cover is too cute!