Slade House, by David Mitchell
This is a little short novel that is also a sort of minor spin-off of The Bone Clocks, which I read last year and half-liked. Slade House I liked entirely; it was all the parts I liked about Bone Clocks, without all the boring parts or unpleasant people. A few characters from Bone Clocks make an appearance, but I don't think it's really necessary to have read it to enjoy Slade House, which is almost entirely its own thing.
In a tiny, ignored London alley, once every nine years a little door appears. Those lonely few who enter find an unexpected country manor house and a congenial companion...until they realize they can't leave. Anyone who discovers what happens at Slade House never leaves to reveal it to anyone else.
It's a creepy haunted-house story, and I liked it. Plus the inside cover is pretty cool too.