Wilkins' Tooth -- Diana Wynne Jones

Wilkins' Tooth is the British title for this book.  Witch's Business is the American.  Want to know how I know that? 

I have a copy of each.  That I bought at the same time.  Because I thought they were different books.

It was an Internet order, okay?  I was in a hurry, I didn't look very closely, I thought I was buying two different Diana Wynne Jones books that I'd never read before.  I was so excited.  (If you don't know this already, Diana Wynne Jones flat-out rules.  Even her less-good books are better than most other stuff.  Recommend the Chrestomanci series and the Derkholm books to Potter fans, The Ogre Downstairs and other juvenile titles to Ibbotson fans, and just recommend Howl's Moving Castle to everyone.  No, I still haven't seen the movie.)

Then, of course, my box of books came and I realized that I was a moron.  Sigh.

Anyway.  This was the edition I happened to read.

Actual plot aside, the highlight for me was Buster Knell, the bully who uses quite colorful language:

"I'll let you off that purple ten pence," said Buster, "if you can get me my blanking Own Back on that blue-and-orange scum.  Only I bet you're too blanking scared."

Frank and Jess are flat broke:

So Frank came dismally home and, after some thought, he and Jess put up a notice on the front gate, saying ERRANDS RUN.  It had been up half an hour when their father came home and took it down.  "As if you two haven't done enough already," he said, "without decorating the gate with this.  When I said no money, I meant no money.  Don't think I'm going to let you get away with immoral earnings, because I'm not."

It was the talk of immoral earnings that gave them the idea.

So Frank and Jess put up a new sign (and pin it to the back of the potting shed instead of the front gate) that reads:

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

It doesn't take long for the neighborhood kids to notice the sign.  It also doesn't take long for Biddy Iremonger, the local witch, to notice a decline in her business.

In most other books, the little old lady that every kid in town knows and fears due to her magical abilities would actually end up being:

A) nice but shy, with no magical powers
B) misunderstood (possibly mentally ill and in need of a good, helpful child to weed her overgrown garden) with no magical powers
C) a bitch, with no magical powers.

But, this is a Diana Wynne Jones book.  Biddy Iremonger is a huge bitch.  With magical powers.