Read: The Art of Nonfiction
Rand's The Art of Nonfiction is not about writing (with) style, it's about the writing process. This “Guide for Writers and Readers” is not a textbook guide on writing, it's a transcript of a lecture series. It's not even written and edited by Ayn Rand, it's written by one of her “students.” The result is maybe less than what an accomplished and skilled author would have produced were he tasked with writing a writing guide for nonfiction texts. Maybe it is more.
If you manage to get past her personal philosophy that bleeds through every paragraph you will receive some very valuable advice on how the process of writing could be organized, on what an orderly method of thinking may look like in order to produce an effective piece of nonfiction.
The actual advice could be summarized on just a fraction of the pages that the book comprises. Yet, the true value of the guide lies in the construction and deconstruction of Rand's own and other's writing to illustrate her points. (Rand is a merciless, arrogant critic. She knows better and proves it.) By deconstructing her own writing, by illustrating her outlining, drafting, and editing process the reader may indeed learn a valuable lesson that the pure abstract advice may have failed to convey.
The examples are out of date, her philosophy and her demeanor may alienate, her advice on the writing process is sound.