As an adult fantasy, this book doesn't quite live up to the standards Diana Wynne Jones has set with her children's books. The book contains several surprising twists, a lot of baby language and kamikaze sex, amongst other things.
The complicated plot revolves around a mage named Mark Lister, whose life is less than satisfactory. As one of the mages who heads The Ring - a secret society of witches who work to keep Britain, and more ultimately the World, safe from destruction - it becomes his duty to consult fellow witch Gladys when he discovers something is tampering in Earth's affairs. A visit to a badly injured girl in a local hospital reveals that another Universe has been creating disasters, in order to steal the ideas that the Ring come up with to save the Earth.
A bus full of witches is sent to the other universe to deal with the problem. The trouble starts when Mark's ex-lover stows away with them, taking her toddler, Marcus.
If you love Diana Wynne Jones's books, this is a good one to add to your collection. If you haven't tried her before, it may be better to start with Howl's Moving Castle, or Deep Secret.