Excellent and very readable book by Alan Garner, British fantasy writer. The plot concerns an alternate reality separate from, but linked to, our own. Children are called into that world from this one and given artefacts, vital to the life of Elidor (the other world). When the artefacts are brought back to this world, they become very mundane objects, but they still have strange effects on electrical objects nearby. The objects, though, can be smelt from the other world, and are being tracked by a pack of nasties. At particularly intense moments the nasties can see our world.
Obviously they don't manage to get their hands on the artefacts, and obvioulsy Elidor survives; the working out of the whole thing, though, is pacey and energetic stuff, and massively enjoyable too.