Drinking midnight wine - Simon R. Green

When Toby Dexter falls for the woman on the train, the woman with the most perfect mouth in the world, he little realises that she isn't quite human: she lives in the magical world that exists alongside our own. And when he follows her to ask her out, he accidently slips from his own world, Veritie, into hers. She warns him that it's a dangerous thing to be a mortal in the magical world of Mysterie and that he must not fall in love with her: she's much older than she looks and mortal must not love immortal. But for Toby, it's too late.
But because this is a Simon Green novel, it's not just a romantic fantasy: in Mysterie, there's big trouble afoot, and before Our Hero can win the hand and heart of his Lady, he'll have to face malevolent immortals, shapeshifting demons and violent gods, all told with Simon's trademark tongue-in-cheek humour and excitement.