Wintersmith by Terry Pratchett

From chapters.ca:
Tiffany Aching is a trainee witch — now working for the seriously scary Miss Treason. But when Tiffany witnesses the Dark Dance — the crossover from summer to winter — she does what no one has ever done before and leaps into the dance. Into the oldest story there ever was. And draws the attention of the wintersmith himself.
Can Tiffany deal with the consequences of her actions? Even with the help of Granny Weatherwax and the Nac Mac Feegle — the fightin’, thievin’ pictsies who are prepared to lay down their lives for their "big wee hag."

Wintersmith is the third title in an exuberant series crackling with energy and humour.