The Essence of Tibetan Buddhism, by Lama Thubten Yeshe

The first part of this small book is the transcription of Lama Yeshe covering for an ill his holiness the Dalai Lama at a conference in France.   It is exceptional in its clarity and humor.

The second part is a bit more complex but still very well written.

The folks at the Foundation for the Preservation of the Mahayana Tradition (FPMT) make this book available as part of the Lama Yeshe Wisdom Archive.    This is quite wonderful: you can read for free online or if you prefer paper, you can obtain several of their books for free (paying only shipping) as part of your quest to understand more about the practice of Buddhism.   This book is part of the Wisdom Archive's starter pack and imho is the way to go.