Many people consider Law’s Empire to be Dworkin’s most important work on law. For those interested in a college level course on the book, I recently listened to this one. It is a 14 lecture course that spends 7 of the lectures discussing Law’s Empire. (The first several of the lectures are on legal positivism and H.L.A. Hart, and the last 4 are on Bruce Ackerman’s constitutional theory.)
At times, I found the course frustrating, but it is hard to know whether that was because of the professor or the theories he is discussing. In any event, I don’t know of any online courses on the subject, and therefore would recommend the course if you are interested in learning more about Dworkin (or the other subjects discussed in the course).