Cliff's Notes version:
Rand's philosophy, highly distilled.
Objectivist philosophers can go on for days on this stuff, but the essentials are easy enough to grasp. The novels are a more entertaining way to pick it up than the nonfiction. You probably read "Anthem" and "The Fountainhead," which are the two major works that get read in schools.
"Atlas Shrugged" is long, dramatic to the point of being allegorical, and breaks for somewhat windy lectures by the characters, including a major stemwinder from the hero near the end, but it describes Rand's philosophy better than any of her other fiction. Also, the plot is very good, and it's a pageturner. Frankly, the first time I read it I skipped Galt's speech to see what was going to happen to the characters.