Zimmerman presents in the book an account of the development of the notion of authenticity in Heidegger. Broadly construed, being authentic means, for Zimmerman, being what one "already is" - namely, removed from egoistic illusions of security and self-control, while remaining the open finitude, the constituting nullity (Ab-grund) that lets Being manifest itself.
By examining Heidegger's works as well as his life course, Zimmerman argues 1) that Heidegger's early works bear an overemphasis on the will, or resolution, of the Self in order for it to become authentic, which presumably results from Heidegger's reaction against dogmatic theology (which promotes conformism); and 2) that the sense of authenticity as releasement [Gelassenheit] is already present even in his early works, so that there is only a "maturation" of the concept of authenticity through time instead of radical change or utter abandonment of the concept.
The book is especially inspiring in 1) that it plays elegantly with the peculiar features of absence / transcendence / openness / finitude and, correspondingly, with the fate of Being to be negated, substituted while dimly revealed; and 2) that it links Heidegger's thoughts to thinker such as Nietzsche, Tillich, Kierkegaard and Schelling, while remaining careful on their differences.
However, Zimmerman perhaps also overemphasizes the negative side of releasement, so that he downplays, now and then, the importance of expression / philosophizing. The words should not be forgotten even if they successfully helped us get to the Word, and this is actually evident from the essential nullity of Being. As a result, his comparison of Heidegger to Zen Buddhism seems less plausible. Besides, although Zimmerman is sensitive to horizontal links between different philosophers, sometimes he fails to pay enough attention to the inherent discrepancies between philosophers he uses to explain Heidegger, e.g. those between Kierkegaard and Nietzsche, so that the comparisons ends up somewhat superficial.