Should a book be structured like an essay? ie intro,body,conclusion?
....with the body being the storyline?
A nonfiction book can certainly be structured that way.
As for a novel, there are some novels with character narrators who address the reader at the start of the book to present a point of view or belief about life which is the equivalent of a thesis statement in an essay.
The bulk of the novel consists of the narrator telling the events of the story that drew him to that point of view and are intended to convince the reader that the thesis is sound.
To take an example, The Great Gatsby
by F. Scott Fitzgerald follows that pattern. It starts with the narrator, Tom, telling the reader about the importance of not judging people until you get to know them. Then he tells the story of how he came to know Gatsby, and concludes the book with a description of the qualities that made Gatsby admirable, even though Gatsby was, on the surface, a con-man, an adulterer, and a poser, obsessed with money and image.
Similarly, Jane Austen's Pride and Prejudice
begins with the thesis "It is a truth universally acknowledged that a single man, in possession of a good fortune, must be in want of a wife." This thesis is eventually proven by the various marriages that take place in the novel.
However, most novels do not have such a character narrator who states upfront what message he/she is trying to convey.
Stories do have premises or morals, lessons which are proven by their events and conclusions, but these are usually not stated upfront. Rather, they must be inferred by the reader. Fully developed stories often have more than one thematic issue that is being explored in the story, and can present several different morals.
Also, a well-developed story presents all sides of the argument, not just evidence that supports the premise. For instance, if you have a premise such as, "Love conquers all," the events in the story will, on balance, prove that love is stronger than the forces which oppose it. However, to avoid appearing too heavy-handed, you should probably include some events where love loses or where an opposing quality (perhaps hate, fear, greed, prejudice, etc.) wins.
Failure to include this kind of balance can make the story seem trite and unrealistic (because everyone knows that in life things are never black and white).