A Tale of Time City, by Diana Wynne Jones (1987)
Vivien Smith was being evacuated from London in WW II when her trip to the country turned out to be much more than she bargained for--she ended up kidnapped by two boys from the future, and whisked off to Time City. There, far from the familiarities of home, she finds herself in a pickle of twisted time, and Jonathan and Sam, would-be-heroes, find they have the wrong Vivien Smith. She's just an ordinary girl, not the powerful twister of time they had hoped to capture.
Vivien can readily accept that the twentieth century was unstable, time-wise; but it's a bit harder to grasp that Time City itself, with its elaborate edifices and artifacts accumulated over the centuries, is about to collapse. To try to keep that from happening, Vivien, the two boys, and a helpful android set off to whisk through the ages, searching for the lost artifacts that will stabilize Time City, and, in a temporally rippling way, all of the past...but someone is to be working against them, and none of their plans are working out....
Oh dear. This is my least favorite Diana Wynne Jones; I had hoped, this second time through it, that I would fall for it, but it was not to be. There is just too much detail. Too much Happening. To many things, and people, and little bits of plot that never coalesce to make magic happen. And time travel-wise, it's a bit of an amusement park ride, rather than a finely wrought immersive experience. I never quite grasped the whole Point of Time City's existence, or why people traipsed around through time...and so, time travel-wise, it didn't engage me, and by the time the Exciting Final Showdown happened, I wasn't all that sure I cared.
Yet. If you like lots of detail, if you don't mind not having a clue for much of the time, if you can appreciate great inventiveness, you might like this one....especially, judging by Stella Matutina's review, if you are a child....I was a grown-up when I read this for the first time, and am still a grown-up, so I never got to read it with the (cliche alert) bright wonder of the child mind. There are bits that almost sing, but.