We've only read the first two thirds of the serialized version so far, but here's the description found on the cover:

A suddenly appearing curiosity shop owned by a small man who might, or might not, be the Man in the Moon; a pair of strange spectacles buried in a fishbowl full of marbles; an old window glazed with sea- green glass found beneath a suburban house; and two adventurous boys who buy the spectacles and climb through the window into a land of goblins, ghosts, and rope ladders that reach to the moon ... Who exactly is Mr. Deener, the fat man who makes magic out of bits of coloured glass, has a passion for glazed doughnuts, and whose seeming twin brother sleeps fitfully in an attic room? And who are the little men that ride out of the forest on windblown sycamore leaves in order to whisper into Mr. Deener's ear? Is Mr. Deener, like a fallen Humpty Dumpty, broken apart? John and Danny need to know. To find their way home they'll have to put Mr. Deener back together again and solve the mystery of the sleeping land - a task that leads them to the pool of reflections in the deep woods and ultimately to a house built of light and magic and memory that sits at the edge of the heart's ocean.