The book is very short. It has only four chapters, book ended by an introduction and conclusion authored by John Swinton. (The expanded 2018 edition also includes a study guide, with chapter by chapter questions for review.)
Two of the chapters were authored by Vanier and two by Hauerwas. Vanier's chapters deal with practical and personal accounts of life within L'Arche. These accounts, though theologically muddled, demonstrate Vanier's real and significant concern for the citizens of our world least likely to be valued. The vision he provides of people with cognitive disabilities being treated as humans, with personalities and flaws like the rest of us, is compelling. Instead of hiding the mentally disabled from view and treating them like patients, L'Arche communities involve co-living with "normal" people who interact with them as neighbors rather than as clients. This process both recognizes the unique needs of these individuals and their distinct value as humans.
Hauerwas' chapters are more theological in nature. As is usual for Hauerwas, they are eminently readable, very pointed, and powerfully prophetic. The theological vision presented in this book is an application of his broader pacific ethic on the structure he outlines in books like The Peaceable Kingdom. This is vintage Hauerwas, both prophetic and conflicted in nature, but distinctly worthy of parsing.
The most significant point of this volume, which is a powerful one, is that among its many ethical concerns, the Christian church must remember the cognitively disabled. Caring for them is a reflection of the Church's gospel witness in the world. Because the cognitively-disabled can often contribute very little to society in terms of economic productivity, they are often sidelined and hidden from sight. To some degree, this is necessary to shield them from becoming spectacles and ensuring they receive appropriate care. However, the treatment is often as bad as the condition, resulting in ostracization and isolation that denies the imago Dei in the mentally disabled -- as if the primary, and perhaps sole, way that we image God is through rational capability (i.e., functional) rather than in our unmeasurable personhood (i.e., ontological). Hauerwas and Vanier draw attention to the insufficiency of this perspective and the need for deeper theological reflection and practical action for the good of Christian communities and the gospel witness of the universal church.
(Few would connect what Hauerwas has to say here and his experiences dealing with Anne's mental illness during his first marriage. But if you take this short little book and line it next to Hauerwas' very pastoral and therefore theological account of his own life [in Hannah's Child] in those 20 years, you'll see why Hauerwas says what he says here and in other places about the way in which the Church ministers to and with those who are differently abled.)