Medical science has reached a stage of development in which the ability to keep a person's body alive well outstrips its ability to maintain worthwhile cognitive functions. Consequently, irreversible comas, persistent vegetative states, and extended periods of unconsciousness during the final stages of an illness are increasingly common. This raises troubling questions for families and doctors regarding what sort of treatment is appropriate in such cases. In response, living wills, durable powers of attorney, and other advance directives have become increasingly popular. Still only a small percent of people draw up such instruments. Planning for Uncertainty: Living Wills and Other Advance Directives\ by Doukas and Riechel provides a useful guide to thinking about these critical situations and offers practical advice on drawing up an advance directive.
The work provides some background for the legal development of advance medical directives, but it's real strength is providing the reader with the opportunity to think carefully and systematically about issues that they might otherwise consider in a jumbled and confused way. However, when all is said and done, it seems that the purpose of most advance directives is to provide assurance to the doctors and family members of an unconscious patient that withholding or withdrawing treatment is a morally acceptable path. In some cases, it can also bind a family to withhold or withdraw treatment or establish obstacles to such a course.
There are numerous books of this sort in print. One may be as good as another. Anyone seeking to draw up an advance directive would be well advised to investigate the state laws that regulate such documents.