Books for understanding Tibetan Buddhism

I have been practicing Buddhist meditation for a few years, mostly from reading translations of the Buddha's original discourses, as well as some Prajnaparamita sutras from Mahayana Buddhism. I've always been intrigued by Vajrayana, but most of the source texts don't seem to have quality English translations, especially the more esoteric Tantric practices. Are there any good texts that go into detail about the principles and practices of Vajrayana, preferably with translations of source texts? So far I've read the Robert Thurman book, which was a bit underwhelming in terms of practical value.