A Well-Timed Enchantment, by Vivian Vande Velde (1990, middle grade--that's the original cover shown at left).
Deanna is not happy to be spending her summer with her French relatives in the middle of the country, with only the cat, Oliver, for company. But she becomes even more unhappy when she accidentally drops her Micky Mouse watch down an old well...it was a treasured memento of the last time her father was a happy member of her family.
And she becomes still more unhappy when two rather snooty elves (although they don't think much of that term) appear, and proceed to scold her for having dropped the watch into a time-displacement vortex (the well is no ordinary well). Unless she can get it back, they inform her, it will alter the middle ages, and, in a domino effect, the entire course of world history will be changed.
So without any choice in the matter, Deanna is plunged back to the middle-ages...but the elves have thoughtfully provided her with a companion--Oliver the cat, in human form.
The middle ages to which Deanna travels is something of a deliberate caricature, with bumbling would-be-knights, a vain and mopish lady of the castle, pigs, greasy chunks of meat, etc. And Deanna and Oliver have no clue how to behave--the elves failed to provide lessons in castle etiquette for Deanna (although they did bestow the gift of medieval French), and Oliver, being a cat, is even more at sea. Over-the-top medieval romance complicates matters, but more seriously, the castle is home to a rather terrifying alchemist who has some genuine magical abilities, and naturally, he's the one who ends up with the watch...
It's all a bit silly, with plenty of deliberate eye-rolling moments, some slap-stick, and flashes of the sharp humor I appreciate in Vivian Vande Velde. The best part of the book, by far, is Oliver the cat--we see through Deanna's concerned eyes as he struggles with being human. Sometimes this is poignant, and sometimes amusing, especially when dogs are mentioned:
"Well," [said Deanna], "you heard Leonard and Baylen were fighting to see whose fiancee is the fairest. It turns out since Leonard lost so badly, he figures his lady must be a real dog, so he wants to replace her with me."
Oliver stopped and stared at her. "Leonard is marrying a dog?"
Deanna sighed. She reminded herself that he had come to her rescue twice already today. She sighed again. "It's just another expression, Oliver." (page 57)
Not only does Oliver add great interest to the story, but he is also the catalyst for change in Deanna. She not a very strong character, being kind of lonely and passive, and she knows it, but her concern for Oliver does, by the end of the book, force her to greater depths of personality. "Will Deanna and Oliver fall in love?" becomes a much more interesting question than the slightly MacGuffiny story of the watch.
I found this a pleasant enough read of the faux time-travel sort (there's no point in expecting historical accuracy), and the cat-loving girl suffering through the blahs of sixth and seventh grade would probably enjoy it a lot more.