Bokar Rinpoche wrote a book, "Tara: the Feminine Divine." Google it. Great stuff.
Also, Bokar Khenpo Lodro Donyo wrote a book about Tara, which is harder to find in English translation, and expensive, when I last looked...it has some info relating specifically to Chogyur Lingpa's terma.
The Khenpo Bros. also wrote a book about Tara, if I recall, which I'm sure is wonderful. And Martin Wilson wrote "In Praise of Tara."
And then there's "The Cult of Tara," which has a great deal of info about Tara, specifically, and Tibetan Buddhist practices, in general.
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