The book is derived from the undergraduate lecture courses Rawls gave at Harvard from the late sixties until the end of his career. Very few in our society have more insight into this complex and challenging period. The book begins with an exigesis of Hume's moral philosophy - its sources and its meaning. The bulk of the book is then, as one would expect, devoted to Kant. The book closes with a study of Hegel. Along the way, all the major schools of moral thought in Europe, especially England, during the era are discussed.
Although I cannot agree with the previous reviewer that Rawls writes`"beautifully" (in fact, I would respectfully suggest that Rawls' literary shortcomings significantly limited the readership of A Theory of Justice, though I would not know how else it could have been written), he thinks magnificently. Rawls was, above all, a profound student of those he calls "the exemplars" of moral theory, and specifically of this period, which is perhaps why he is simply the BEST teacher of this material. His insights into Hume, Leibniz, Kant, and Hegel will forever change your way of reading these giants. Rawls displays bona-fide, hard-earned insight.
Because Rawls thinks and writes in a way the encompasses a plurality of levels of explanation and speculation, the study is as enriching for the professional as for the student with modicum of basic knowledge. If you read one book on this subject, read this one. Also, Rawls is very good about documenting his sources, which really helps if you desire to go further.