The Master and Margarita by Mikhail Bulgakov
This is a brilliant book, which I remember reading in the Lake District around the time of Under The Iron Sea. It was originally published in Russian, but I read an English translation. It's a very dark and humourous magic realism tale. It's basically about Satan coming to Moscow in the form of a magician, with an entourage of really bizarre animals and ghostly figures - like this terrifying cat which constantly changes shape. They go around terrorising Moscow and bringing down all the pompous figureheads of Russian society. It's very weird, and at times quite scary, but really funny as well.
I think the whole thing is meant to be an analogy for Russian society at the time it was written, in the 1920s and 30s. It was banned when it was written and wasn't even published while Bulgakov was alive.
The novel is very leftfield in its style and the scope of its imagination. It has this whole side story about Jesus being tried and crucified, so it will leap between Jerusalem and Moscow and obviously through time as well. It probably sounds like it could be quite dry and earnest, but it's really not at all. It's essentially a kind of ghoulish adventure, that's also quite ludicrous and funny. It really is a fantastic read.