It's hard to fairly judge a story when you've first got to know it in a different form. The few films that I've ever walked out of have been adaptations of books that I had read. In this case I knew the film before I read the book, and I couldn't help judging it in the light of the film.
In the world of the story, witches are a part of everyday life, but much rarer than they once were. Kiki, the daughter of a witch, has the choice of becoming a normal girl or a witch, and chooses the latter. Witches' skills have grown fewer over the years. Though Kiki's mother can both fly and make potions, Kiki herself has only learnt to fly. Her one other piece of magic is that she can talk to and understand her black cat Jiji, born at the same time as Kiki. In the year when she reaches thirteen, the custom is for a witch to spend one year living independently in a strange town. Kiki leaves the little town between wooded mountains where she grew up and sets off southwards, towards the sea. Following a river she comes to a large coastal town, with an improbably high clocktower, and decides to try making that her home for the year. The cool reception from the townspeople is a shock to her; but kinder treatment from a baker, Sono, persuades her to stay and earn her living by delivering parcels for the townspeople.
So far, those who know the film should find a lot that they are familiar with. Many of the strengths of the film come from the book. Kiki and other named characters are much the same in both. Kiki's parents are the same combination of stricter mother and more indulgent father. Jiji has the same cautious outlook on the world, but also a more childish side in the book. The themes are very similar; but Miyazaki makes the film more a story of a child coping with having to rely on their own emotional resources. The book is far more episodic: it follows the year that Kiki spends in the town with a series of seasonal episodes. Miyazaki selects a few of these (often radically changed) and puts them together to tell a story which does not attempt to fill the whole year. The film also shifts the emphasis towards realism, with its loving creation of a beautiful but plausible Scandivian town. The book allows itself a lot of deliberate comic absurdity, of which very little gets into the film (only the episode where Jiji must pretend to be a stuffed cat, I think). For instance, when the mayor discovers that the clock is broken on New Year's Eve, he wants to 'borrow' a gear from the clock of a nearby town, which has the same mechanism.
As soon as Kiki landed on the clock tower, the mayor spoke agitatedly, 'The fact is, this largest cog wheel of this clock is broken, you see? ..... If you cross three mountains west of here, the town over there ..... You could make an appropriation and come back to us, hmm? .... um, um, it's really urgent. ..... hmm? hmm? hmm?'
'Appropriation?' Kiki asked round eyed.
In response the mayor shrugged his shoulders and said in a small voice, 'In other words, just for the time the clock is sounding midnight, borrow it without telling anyone ..... is what I mean.'
'In other words, steal it?'
'Shhh! That's a bad word. That's not a word that a girl should be using. Definitely, definitely "borrowing" is what we want to call it. After all, we'll give it back when we're done."
By some accounts, Kadono was not happy with the film's changes to the book. But the character of the story is not massively changed beyond what I have described. Those who like the film would probably find the book interesting too.