This is a fantastic book- if you are a Joan Aiken fan, buy it immediately. Rather than using the mythical, historical backgrounds of the Wolves... series, this is set in modern England- an England which has been overrun by monsters- and the adventures and struggles of two cousins to escape the monsters and the more sinister threats which follow them, and their attempts to unravel the mystery of where it all went wrong.
Like all Aiken's best work, there is a deeply scary, nightmare thread running through this book, which probably makes it unsuitable for impressionable younger readers, but makes it ultimately more thrilling and involving for older readers and adults.
The monsters are especially entertaining- drawn from Lewis Carroll, ancient mythology, and even Monty Python, they are scary and funny at the same time. Although the book reaches a satisfying conclusion, not all the more peculiar loose ends are tied up, leaving room for re-reading and re-interpreting.
A brilliant book.