Blood Meridian: Or the Evening Redness in the West by Cormac McCarthy. His best book and, IMHO, the most recent unquestionable addition to the Western Canon. It is one of the most frightening things I've ever read, and by far the goriest, though it all resonates with a mythic sort of purpose. Additionally, his descriptive ability is unique and reliably mindblowing.
Thus Spake Zarathustra by Friedrich Nietzsche is... important. By the end section, I can't help but feel the intensity of Nietzsche's pitch and be vitalized by it.
So, take your pick: fiction or philosophical poetry! Whatever you're in the mood for.