The Fountainhead is all Objectivism, all the time. Objectivism is Ayn Rand's pet philosophy, and it urges people to act in a way that is (according to the Ayn Rand Institute, so it must be true) selfish.
If you work hard and with absolute self-interest, you'll eventually succeed, states Objectivism. And hey, guess what? The plot of The Fountainhead is...a guy works hard with absolute self-interest and eventually succeeds.
That guy is Howard Roark, and The Fountainhead follows his life from a struggling, uncompromising architecture student to a successful, uncompromising architect. Ta-da! Objectivism FTW.
Sure, there's more to the novel than that, but the central focus is on how Roark uses the tenants of Objectivism to end up happy. It's a novel that charts a certain philosophical ideal, and because of that, it's totally in the "philosophical literature" camp.