This is an early portal fantasy that I was fond of as a kid. On the reread, it's a whole lot shorter than I remember, and more unrealistically optimistic about the notion of overthrowing an evil king with zero bloodshed. But it stands up pretty well anyhow.
Carrie and her cousin Digby meet up for their regular summer vacation in the nowhere town of Pawthany-on-Ilse. This year, however, a stained-glass shack has popped up, labelled "Mural Shop". The (only somewhat hobbit-like) Mural Master invites them inside, along with two other visitors, and -- if I say they wind up painted into a corner, you'll hit me, right? Magic country, greedy king, beautiful daughter, prophecy, evil magician, monsters, adventure, true love, all in due order. (Note: prophecy is not actually prophetic and its import is rather nicely explained.)
The magic land is the best part here (okay, except that it's named "Pawthania", but ignore that). It's vivid, full of high-concept scenery, and surprisingly not beholden to D&D fantasy tropes. (Even the short hairy-footed Mural Master turns out to be his own kind of thing, and certainly no Bilbo clone.) The brushstrokes (sorry) are broad but distinctive. Same goes for the protagonists. The group is both less white and less stereotyped than you might imagine for 1974; Tonio Dias is the storyteller -- favoring Poseidon, rather than anything culturally Hispanic -- and Leo Lopopolo (of some Caribbean ancestry) winds up the de facto team leader. Carrie could be described as "spunky" and she screams a couple of times, but everybody gets their turn in the doing-things spotlight. I suspect the book never got a second edition, so you'd have to go trawling the used-book networks, but I recommand it anyhow.