Before I type some recommendations, I would make a suggestion.
One of the ways in which Mahayana Buddhism, as presented by Tibetan Buddhism is classified is in 3 parts- suttrayana, vajrayana and tantrayana. The order is important. Think of the former ones as sort of a pre-requisite. Therefore, without knowing sutras well, the philosophising or reasoning of vajrayana would not be effective and without this, the esoteric practices of tantrayana wouldn't be helpful. In fact, they can particularly be harmful.
([Original Author], [book title abbreviated], [translator/editor])
- Nagarjuna's MMK "Nagarjuna's Middle Way" by Siderits and Katsura.
- Vasubandhu's ADK (and the auto-commentary) "The Treasury of Abhidharma" by Gelog Sangpo and Dhammajoti
- Dharmakirti's PVT pt2 by Tillemans.
- Candrakirti's MMA "The Emptiness of Emptiness"by Geshe Wangchen and Huntington.
- Santideva's BCA "The Guide to a Bodhisattva's way of Life" by Stephen Batchelor
- Tsong-Khapa's "Ocean of Reasoning" by J.L. Garfield
- Tsong-Khapa's "The Great Treatise on the Stages of the Path to Enlightenment" by The Lamrim Chenmo Translation Committee (which comprises of some of the best Buddhist scholars from west)
I would highly recommend reading commentaries of the books also (from correct scholars of course).
Other than original texts translated into English and commentaries, I would also suggest the following list of books:
- "The Words of my Perfect Teacher" by Patrul Rinpoche.
- Works of HH Dalai Lama, written with Jeffrey Hopkins; especially "Awakening the Mind, Lightening the Heart"
AND of course, please be very careful about the person from whom you pick up Dhamma/Dharma teachings.