Complete failure to take itself seriously. Works very well. The kinetic-energy-charged magic ring is a great touch, and I grinned at Elidee the pixie's name...
But "429B Baker Street"? That would be in the middle of the boating lake in Regent's Park. Numbers as high as that are unusual in British addresses, and Baker Street's numbers didn't go up to 221 when Conan Doyle was writing.
ISBN 9781841494012.