The Romantic Manifesto is a pleasant switch from the usual Ayn Rand that we've all come to know and love. It is part of her non-fiction series (meaning, in brief, that it is blatant about its assertions instead of disguising them as cheap plot devices), coming after Capitalism: The Unknown Ideal. It deals primarily with objectivism (obviously) and how it relates to aesthetics — the study of beauty, art, and value.
The biggest question she attempts to answer within her essays centers around the process of the artist imbuing art with meaning. To Rand, art is a recreation of the One True RealityTM that is then distorted by the artist as a medium for communicating their personal conception of beauty. She often refers to the difference between the reaction one has upon seeing a beautiful woman with a cold sore and the reaction upon seeing a painting of the same woman. In the first case, the cold sore doesn't particularly stir any emotion, but in the second case it is a statement along the lines of "beauty is only skin deep."
And so on. People familiar with Rand's particular style of prose and her other works won't find many surprises in The Romantic Manifesto. The book is no where near as fanatical or dogmatic as some of her other works. It seems as if more of the human part of Ayn Rand wrote these essays than the cold, novelist Rand. Though she references her works of fiction (as ideal instances of literature, no less!) within the text, someone unfamiliar with Atlas Shrugged or The Fountainhead would be comfortable with The Romantic Manifesto.
Being a recovered Ayn Randroid myself, The Romantic Manifesto is the only remaining book of Rand's that I take seriously. Her attack of modern art is, frankly, inspired (if pedestrian) and easy to replicate when the need arises. On the other hand, it has little to contribute to the state of aesthetics as a whole, as Rand is somewhat notoriously strong on definition and short on justification. Some of the less amusing parts include a supposedly complete enumeration and classification of all possible forms of art, a full-fledged rant on how communism kills romantic expression, and how art can only exist in a laissez-faire free market.
As with everything Ayn Rand, skip it unless you're part of her cult.