What is the whole entire book about from beginning to end?
That's a pretty tall order, one that can't be completely fulfiled in theis format. However, here an excerpt from eNotes One Page Summary. You can find out more about themes, characters, etc., by visiting the link below:
"While avoiding the hyper-realism of so many recent novels for children and young adults, Tuck Everlasting nonetheless gives a believable and loving portrait of real people in a very difficult, if somewhat fantastic, situation. Winnie Foster is neither an angel nor an anti-hero. She's simply a young girl with strengths and weaknesses; she is basically good, but far from perfect. The Tucks, at first glance, come across as little more than endearing, slightly mysterious country bumpkins, but we quickly realize, as Winnie does, that there is much more to them than is immediately apparent. Angus and Mae Tuck are unlettered, but wise in their own way, and they have much to teach Winnie about life.
The central event of the novel—the Tucks offering Winnie immortality—is both intriguing and problematic. Should she accept the offer? Should she consider marrying Jesse Tuck and coming to live with them? Whether she herself chooses to live forever or not, should she keep their secret? The Tucks' slovenly but free lifestyle stands in obvious contrast with everything Winnie hates about her parents' and grandmother's prim, fenced-in, middle-class existence. Winnie, however, must decide whether the freedom the Tucks represent is right for her. This decision is made even more complex when the unnamed stranger in the yellow suit makes an appearance and, in his overwhelming desire to gain control of the fountain of immortality, threatens both the Tucks and Winnie. His death at Mae Tuck's hands and her impending execution on a charge of murder add the final components to what is already a difficult and compelling moral problem. "