A 1991 children's fantasy (and borderline mystery) novel by Patricia C. Wrede. The world is Regency England, only with a touch of magic. At times, there are too many people with too many names to easy keep track of, and the climatic scene goes on a bit long (by which I mean roughly 40 pages, though I suppose it could count as two or three scenes), and is followed by twenty pages of exposition. Still, quite a good story, with a bit of humour in the mix. The main character notes several times that it seems suited to a Drury Lane comedy.
I like to ruin people's fun, so beware upcoming spoilers!
Kim is a seventeen-year-old (or thereabouts; she can't quite remember for certain) street waif posing as a boy (being a girl can be unpleasant). She's hired to spy on a magician. Sneak into his cart while he's performing and just see if there's a certain bowl. She doesn't even have to steal it, and the toff will pay her five pounds. Sweet.
However, when she does this, she discovers the magician she's spying on isn't a mere sleight-of-hand kind of guy, but someone with real magic, including real magic traps to stop thieves. Eek!
Caught by a wizarding dude, Kim finds herself not only not arrested, but actually invited to join him as his stage assistant. Largely due to curiosity about a wizard is going travelling around performing magic shows, she hesitantly agrees, much to the annoyance of Mairelon's paranoid comrade, Hunch.
Quite quickly, the three are out of London, though only after Kim collects her five pounds--at Mairelon's advice--and gets attacked by the toff who's unhappy with her report. Oh, and some rough blokes she knows are after her for some reason, giving her all the more reason to skip town.
Out on the road, however, they're visited by a nobleman, Lord Shoreham and Kim overhears a fair bit of what's going on. Mairelon, it seems, is actually Richard Merrill, former member of the Royal College of Wizards. Five years ago, someone stole several very valuable magical artifacts from the college and arranged it such that Merrill would get blamed. Since then, Merrill has been travelling as Mairelon, attempting to recover the pieces. He's managed to acquire one so far (the bowl Kim was supposed to scout out). He now has a clue as to where one of the platters is.
Mairelon and Kim attempt to steal the platter (as do many others the very same night), only to find someone has already replaced it with a fake.
Well, following a bit of gallivanting around, a total of four fake platters come to light, and quite quickly our heroes find themselves in a comical confrontation with about eight different groups of people, most of them after the platter (though a few merely clueless comic fodder). The platter is finally revealed, as are multiple other pieces of the stolen artifacts, and the original thief is brought to light.
Oh, and it's discovered that Kim has a knack for magic. So Mairelon decides he'll train her as a wizard. Hip, hip! Hooray!
If you liked this book...
Firstly, there is a sequel to Mairelon the Magician, though I have not yet read it: Magician's Ward. It features Mairelon and Kim, as before, but beyond that, I know little of it.
In addition to that, however, you should try Sorcery and Cecilia or the Enchanted Chocolate Pot and its sequel, The Grand Tour, both by Patricia C. Wrede and Caroline Stevermer.. You may want to try the Lyra series by Wrede (Shadow Magic, et al) and perhaps the The Dalemark Quartet by Diana Wynne Jones. I could recommend The Wolves of Willoughby Chase, but that's completely magic-free and probably for slightly younger kids.