A magical adventure begins for Tom when he "finds himself named one of the Guardians, directly responsible for the safety of the Castle treasure. His strange but congenial co-Guardians are the touchy, bearded Hermit and an airily commercial witch, Cassandra Peach. And there are others--sly, secret and powerful--who wish the Guardians ill and the treasure lost". (WorldCat)
This is a strange little book. I remembered getting it from the library many times as a child, and in my project to collect as many of the books I loved as a child as possible, I ordered this and just got my copy and re-read it. It's really well written, and has so much going for it, that I wished it could have been a bit longer and a bit deeper.