Witch's Business (Chrestomanci Books) / Diana Wynne Jones

When Jess and Frank's father stops their allowance for four months, desperate measures are necessary. Jess's friends expect her to pay her share for things, of course, and, worse, Frank owes money to the bully Buster Knell. So they decide to start a business. They make a sign:

OWN BACK LTD.
REVENGE ARRANGED.
PRICE ACCORDING TO TASK.
ALL DIFFICULT FEATS UNDERTAKEN.
TREASURE HUNTED, ETC.

At first the only response they get is from adults who laugh at them. But before long they find themselves working for their first customer -- Buster Knell, of all people. One thing leads to another leads to another, and soon their business has whirled out of control in ways they never could have imagined. What's more, the local witch, Biddy Iremonger, sees Own Back Ltd. as competition to her business, and when Biddy is angry, there's no end to the trouble she'll cause. . . .

Witch's Business, Diana Wynne Jones's first novel, has all the humor, action, inventiveness, and surprises that have established her reputation as one of the finest fantasy writers.