Tuck Everlasting gets all the glory because it's about the Big Topic of Death (and may be the last of Natalie Babbitt's books still readily findable in print editions as well as digital ones, as far as I can tell), but this heavily symbolic and enjoyably odd fairy tale is still my favorite. (As a kid, I recommended it to my mother, who wound up reading it to her fourth-grade class every year until she retired from teaching.)
Like so many Natalie Babbitt books, this one has a strong central moral message that I didn't even realize I was absorbing as a kid, and that feels clever to me as an adult: In a broadly medieval-ish kingdom, the court casually disagrees over what food should be used to illustrate "delicious" in a dictionary that communicates concepts with examples. Except the casual disagreement turns cantankerous, and before long, a young boy is dispatched to poll the kingdom and settle the matter, which just causes upset across the kingdom.
There are magical and fairy-tale aspects to all of this, but it reads much like a James Thurber novel (The Wonderful O comes to mind) in terms of introducing the magical without it being a central aspect of the story. Really, this is a book about how easily fights start, how superficial wars can be, what really unites humanity, and how easily one malevolent person with a hunger for power can manipulate self-righteous, self-important, and intolerant people. This book is as timeless as any fairy tale, but it seems particularly relevant at the moment.