That depends how "Left Hand Path" is defined. Increasingly and especially on the internet, practices which really have no substance behind them are called "Left Hand Path" because the practitioners just like the sound of those words and the history behind them. When the symbols are stripped away, what is usually going on is something that is closer to the RHP or else a kind of fantasy game without a clear objective driving it (usually a lot of immature drama and "rivalries" though).
This has happened because, in the last twenty years, the term "Left Hand Path" has become unmoored from the Eastern tantrism and Western Satanism with which it was once associated. In those contexts, it had internally coherent definitions; the Left Hand Path was about reversing the flow of universal energy in the Eastern approach and it was about individuation to the point of self-sovereignty in the Western approach. There is some crossover between these approaches.
Nothing replaces experience and a living encounter with someone further down the road so that certain qualities can be apprehended in a way which empowers those same qualities within the initiate. With that caveat, these are the books which I would say give the best overview of what's really going on in the Western Left Hand Path:
- Lords of the Left-Hand Path: A History of Spiritual Dissent by Stephen E. Flowers
- The Basic Writings of Nietzsche and The Portable Nietzsche by Friedrich Nietzsche (trans. by Walter Kauffmann) and Nietzsche: An Introduction to the Understanding of His Philosophical Activity by Karl Jaspers
- The Gurdjieff Work by Kathleen Riordan Speeth
- The Psychology of Man’s Possible Evolution by Peter D. Ouspensky
- Living Thelema by David Shoemaker
- Energy Magick of the Vampyre by Don Webb
- Aletheia (3 vols.) by Tapio Kotkavuori
Nietzsche is the key to everything. The distinction between the unblinking cosmos and the Master's psyche, and What the Master's psyche can transform into through self-overcoming, is the philosophical basis of the entire Western Left Hand Path. Crowley translated the "Prophet" Nietzsche ("Prophet" wasn't a title Crowley threw around lightly) through the lens of Victorian magic and orientalism. The best introduction to Thelema is Shoemaker's book. Gurdjieff recognised that "day-to-day" consciousness is the product of external environment and that war had to be declared on this robotic thinking if the Master were to surface. Ouspensky's short book is the best introduction to Gurdjieff and Speeth's book directly applies the Work. Flowers is good for an overview of the Left Hand Path by looking at the way it's surfaced in history.
Vampyre is Webb's best book and shows how the Words of the Temple of Set are keys to power in one's own life. It could be substituted with the ONA's NAOS (original only, not one of the bastardised versions) if preferred.
Aletheia is a memoir of a life spent inside one Left Hand Path school. It shows the relationship between theory and reality.
I'd also suggest two novels and one film. One is Oscar Wilde's aesthete novel
The Picture of Dorian Gray which communicates everything noble in LaVey's movement but with far greater beauty (read Wilde's
De Profundis if you want to see where it can all lead without something greater than chasing highs). The second novel is Arthur C. Clarke's
Childhood's End which depicts the racial memory of a being that empowers magic in some and madness in others.
2001: A Space Odyssey is the loose film adaptation of this same idea and considers the weirdness of human intelligence and why it should have arisen at all.
After all of that, if you want further reading, these are cultural specific lenses and other areas of investigation:
- Northern Magic by Edred Thorsson
- Aghora: At the Left Hand of God by Robert E. Svoboda
- The Yoga of Power by Julius Evola
- Metapolitics by Peter Viereck
- The Magick of Aleister Crowley by Lon Milo Duquette
- Ogam: Weaving Word Wisdom by Erynn Rowan Laurie
- On Becoming an Alchemist by Catherine MacCoun
- A Secret History of Consciousness by Gary Lachman