The Tibetan/Vajrayana is relatively small on a global scale—probably about 10% of Buddhists—but is disproportionately influential in the US due to the Tibetan diaspora which found refuge here following the Chinese conquest of Tibet, as well the charismatic influence of the Dalai Lama. The Tibetan tradition is also interesting in developing far later than the Theravadan and Mahayana, around 1000CE, and incorporates both Indian and Chinese elements as well as older Tibetan practices. And not incidentally, our own University of Virginia has one of the best Tibetan studies programs in the West, and 45 minutes south of Charlottesville, the Serenity Ridge Retreat Center is the international headquarters of Lingmincha International, a large organization of Bön practitioners.

Suggested reading:

• Khenpo Karthar Rinpoche. "Vajrayana Explained" (October 2019, adapted from 2007 text; pretty dense...) https://www.lionsroar.com/vajrayana-unpacked/
• Judy Lief. "Journey to Awakening." (May 2013) https://www.lionsroar.com/journey-to-awakening-may-2013/

Podcasts

Tibetan history and Buddism. https://anchor.fm/rasa-phonya/episodes/Tibetan-History-tied-up-with-Buddhism-ekfbau