'The Birth of the Clinic' is one of four major works by Foucault during his 'archeaological' period. The works are 'archeaological' in the sense that they dig beneath the surface of history; beneath the major events, discoveries and key figures, in order to locate the effect of shifts in discourse and how these transformations in ways of thinking and speaking provide the condition by which the surrounding world and goings-on are made sense of.
This book is very difficult to read. Only those who are familiar with Foucault beforehand should consider reading this book; it requires being familiar with structuralist lingo and ideally some knowledge of medical terminology beforehand. The book takes you on a journey, moving from France and its medical discourse during the 1600s, through to the 1900s and glimpses at the 20th century in its conclusion.
It is effectively a book about how the subject of medicine and clinical practice has undergone certain perceptual transformations in relation to changes in discursive practice (ways of speaking, thinking and accordingly, performing). Questions such as what is the ideal environment for the patient to recover from illness, and what is the fundamental nature of disease have been answered very differently in differing periods. Foucault tries to identify 'the gaze'; the way that the object in question (the patient, disease etc) is treated, and the varying modes of classifying, ordering, relating, distinguishing and so forth are used in address of this object, and how each gaze transforms, opening the possiblity for another to take it's place as a dominant discourse for clinical practice. To this extent it is a challenge to positivism, and modern attempts to stabilise the core of 'man' as an object of enquiry with a fixed identifiable nature (e.g. as in psychoanalysis).
If this seems a bit heavy going, believe me, you will not want to read the book without delving into a book or website about Foucault's thought first. If you're willing to dig through this often heavy going, but not especially long book (about 250 pages), you may well get some interesting insights out of this work.
To summarise then, this is a very thorough work, well researched, but it is difficult, so be well prepared. Read if you are interested in Foucault's methodology, but don't start here, start with an intro to Foucault. The stanford uni press webpage has a good intro to Foucault's thought by Gary Gutting. Barry Smart has written a good intro too.