|The Buddha at Sarnath|
I was an early (1974-1982) student of Chögyam Trungpa Rinpoche, who left the community not long after Thomas Rich was appointed Vajra Regent, but who's continued to have friends in the community and to practice under other teachers in both the Tibetan and Theravadin traditions. My approach has always been to strive to balance practice and study.
What's been most helpful to me in recovering from some of not just the challenging social experiences but also imbalanced ways of practicing I developed while involved with Vajradhatu/Shambhala has been to look afresh at the very earliest teachings and practices of the historical Buddha, seeking to ground the exalted practices of Vajrayana in a much deeper “in the marrow” living of the 4 Ennobling Truths, Eightfold Path, the 4 Establishments of Mindfulness (Satipaṭṭhāna) and so on.
I reference Vajradhatu/Shambhala since that's the Tibetan group I'm most familiar with, but these resources should be of equal benefit to those who've left Rigpa and indeed to anyone wanting to get a sense of just how different the Buddha's teachings and approach were from the many forms of Buddhism that came later. To be clear, this is not to say that the early teachings are better or purer, but rather that they're the foundation for all that came later.
An additional benefit of some exposure to the earliest strata of teaching and practice is that one gets a strong sense of the ethical, experiential and eminently pragmatic flavor of the Buddha's approach. As with Jesus (but perhaps to an even greater degree) one realizes just how much time and effort has gone into blunting the radicalism of the founder's teachings (starting right after his death and continuing up to the present), and that much of what's popularly represented as being his teachings is in fact diametrically opposed to them.
An essential first step for those who missed it (most practitioners, in my experience) is to understand where Tibetan Buddhism fits in in the broader Buddhist context.
Foundations of Buddhism, by Rupert Gethin: a concise college text that is the best overview of Buddhism I've found. It's a standard college textbook - skimming some sections is just fine. If time is tight, substitute Buddhism:A Very Short Introduction by Damien Keown. Tricycle magazine also offers a good Buddhism for Beginners site that's a more-than-worthy alternative to (or supplement for) such books.
Next, please read Freedom from Buddha Nature by the eminent Thai forest tradition scholar monk Thanissaro Bhikkhu to get a sense of just how different "basic goodness" (aka Buddha Nature) is from the teachings of the historical Buddha. A related article by the same author on the oft-quoted (but rarely read and even more rarely undestood!) Kalama sutta is also strongly recommended for the way it contrasts the Buddha's careful and pragmatic ideas for what constitutes authority and trustworthiness with the reverence for "lineage" that's so pervasive in Tibetan Buddhism.
Equally valuable is the very concise Steps to Liberation: The Buddha's Eightfold Path. These two books together are a kind of "pith essence" Cliff's Notes summary of foundational Buddhist teachings that are meant to be put into practice with immediate benefit.
A Meditator's Life of the Buddha by the great scholar-practitioner Bhikkhu Anālayo uses the key events of the Buddha's life to inspire us to truly walk in his footsteps. His Satipaṭṭhāna Meditation Practice Guide with its accompanying guided meditations is the clearest and most profound meditation manual I've found.
Good Kamma, Bad Kamma - What Exactly is Kamma? by Bhante Dhammika is a concise yet complete guide to perhaps the most thoroughly misunderstood teachings of the Buddha: those on karma.
Diving into the Suttas
In the Buddha's Words, translated and introduced by Bhikkhu Bodhi. Very few Buddhists have read much of the Buddha's own teachings, which is understandable given the ancient language and intimidating size of the Pali canon. Bhikkhu Bodhi, the most eminent translator of these teachings, offers this concise and accessible collection, organized by topic. The Buddha's clear-headed pragmatism and wisdom shine through on every page.
The Big Picture/Seeing the Traditions in Context
What the Buddha Thought, by Richard Gombrich, sheds light on the unique genius of the Buddha while also making it clear how later traditions accidentally and willfully misconstrued his teachings. Illuminating, fascinating and essential.
The Tibetan Book of the Dead:A Biography and Prisoners of Shangri-La by the eminent scholar Donald Lopez are invaluable for understanding the “lenses” of preconception, Romanticism and sometimes fanciful thinking through which the tradition has made its way to the West. These are scholarly books that read like compelling mystery novels.
The Making of Buddhist Modernism, by David McMahan provides an erudite yet accessible overview of the key ways ancient Asian traditions have interacted with and been changed by their encounters with modernity. As with Gombrich (but to an even greater degree) this book will be a revelation to anyone immersed in one or more forms of Buddhist practice about the broader context of their practice and the often hidden assumptions and biases inherent in all forms.
American Dharma:Buddhism Beyond Modernity, by Ann Gleig, builds on McMahan's work but is much more up-to-the-minute in illuminating generational differences and current hot topics in a diverse range of Western Buddhist communities. Truly essential reading for anyone practicing Buddhism in the West.