Of course 6 stars is akin to having an amplifier that goes all the way up to 11, but still it is worth 6 stars.
The most essential Trungpa books, imho, are Born in Tibet, Meditation in Action, and this title, Mudra. And the Sadhana of Mahamudra, if you can get your hands on it (his group's essential Sadhana).
Born in Tibet is a nuts and bolts history of Trungpa growing up as a tulku or recognized reincarnate lama in Tibet, and of his escape from Tibet. It's good in terms of placing his early life in historical perspective.
Meditation in Action is sparse, simple, profound, and rivaled only by Zen Mind, Beginners Mind, in terms of the mid-century Buddhism revival in America. Meditation in Action (and Zen Mind, for that matter) is a book you can give to any friend curious about Buddhism without having to include a lot of explanations. Pure, not watered down, but not too tricky either.
And Mudra is perhaps the most personal expression of the heart of this fascinating teacher (purely in my personal, uneducated, unsophisticated opinion). It contains two critical Mahamudra texts, followed by Trungpa's very personal poetry, which combines religious expression with lay expression.
I don't know to what extent Trungpa was influenced by his association with Ginsberg in terms of his poetry, or by Suzuki Roshi in terms of the ox-herding pictures, but it is a somewhat irrelevant question - those two may have kick-started him the in the direction of his "play," but what is on the page is all Trungpa's own.
Being able to carry this anywhere and dive into it anytime on a Kindle or Kindle-enabled device like my iPod Touch just blows me away.
I was never a big fan of carrying around my entire music collection. But being able to carry around a sizeable chunk of my collection of treasured books is great. Some jewels to make up for the pulp fiction that is my entertainment.
BTW the two Mahamudra texts just blow away 300 page tomes on Mahamudra and Dzogchen, imho, the more I dwell on them the more I realize how much is in them. If you are from the school that more is better, have some 300 page tomes as well, but if you need to get to the point quickly, only Hitting the Point in 3 Words (Tsik Sum Ne Dek) is more concise.