Drinking Midnight Wine - Simon R. Green
The protagonist of Drinking Midnight Wine is Toby, who lives a very tedious life working in a bookshop and only clambers out of his rut one evening on his return from work, when he decides to wait out a rainstorm at the train station and witnesses something he shouldn't have seen. Although he's been besotted with the woman who often sits in the same railway carriage, he hadn't realised that she was much more than she appeared, until he follows her from the real world of Veritie into the realms of Mysterie.

There, it seems, a plot is afoot to merge the two worlds together, which will ultimately lead to the destruction of both. Toby finds himself dragged along to both investigate what is going on and ultimately to try and thwart it. It's an entertaining enough book, particularly if you're playing spot-the-real-identity for some of the characters, but I can't say I would read it again.