The First Law School Casebook:
The first law school casebook was Christopher Columbus Langdell's 1871 book, A Selection of Cases on the Law of Contracts, "Prepared for Use as a Text-Book in Harvard Law School," and published by Little, Brown (since bought by Aspen) in Boston. Although Langdell is often classified today as the archetypal formalist, for whom every question had a pure and doctrinally correct answer, it's actually hard to see that in his casebook. Langdell's contracts casebook was just that -- a selection of cases, without any commentary at all. The only commentary on his goals appears in the preface, in which Langdell explained why he thought the case method might work and why he thought it was helpful for law professors to teach from casebooks instead of treatises.

  Most law school casebooks today are combinations of pre-Langdell treatises and post-Langdell selections of cases. Still, it's pretty interesting to read the preface to Langdell's book to get a feel for his thinking:
  I CANNOT better explain the design of this volume than by stating the circumstances which led me to undertake its preparation.
  I entered upon the duties of my present position, a year and a half ago, with a settled conviction that law could only be taught or learned effectively by means of cases in some form. I had entertained such an opinion ever since I knew any thing of the nature of law or of legal study; but it was chiefly through my experience as a learner that it was first formed, as well as subsequently strengthened and confirmed. Of teaching indeed, as a business, I was entirely without experience; nor had I given much consideration to that subject, except so far as proper methods of teaching are involved in proper methods of study.
  Now, however, I was called upon to consider directly the subject of teaching, not theoretically but practically, in connection with a large school, with its more or less complicated organization, its daily routine, and daily duties. I was expected to take a large class of pupils, meet them regularly from day to day, and give them systematic instruction in such branches of law as had been assigned to me. To accomplish this successfully, it was necessary, first, that the efforts of the pupils should go hand in hand with mine, that is, that they should study with direct reference to my instruction; secondly, that the study thus required of them should be of the kind from which they might reap the greatest and most lasting benefit; thirdly, that the instruction should be of such a character that the pupils might at least derive a greater advantage from attending it than from devoting the same time to private study.
  How could this threefold object be accomplished ? Only one mode occurred to me which seemed to hold out any reasonable prospect of success; and that was, to make a series of cases, carefully selected from the books of reports, the subject alike of study and instruction. But here I was met by what seemed at first to be an insuperable practical difficulty, namely, the want of books; for though it might be practicable, in case of private pupils having free access to a complete library, to refer them directly to the books of reports, such a course was quite out of the question with a large class, all of whom would want the same books at the same time. Nor would such a course be without great drawbacks and inconveniences, even in the case of a single pupil. As he would always have to go where the books were, and could only have access to them there during certain prescribed hours, it would be impossible for him to economize his time or work to the best advantage; and he would be liable to be constantly haunted by the apprehension that he was spending time, labor, and money in studying cases which would be inaccessible to him in after life.
  It was with a view to removing these obstacles, that I was first led to inquire into the feasibility of preparing and publishing such a selection of cases as would be adapted to my purpose as a teacher. The most important element in that inquiry was the great and rapidly increasing number of reported cases in every department of law. In view of this fact, was there any satisfactory principle upon which such a selection could be made?
  It seemed to me that there was. Law, considered as a science, consists of certain principles or doctrines. To have such a mastery of these as to be able to apply them with constant facility and certainty to the ever-tangled skein of human affairs, is what constitutes a true lawyer; and hence to acquire that mastery should be the business of every earnest student of law. Each of these doctrines has arrived at its present state by slow degrees; in other words, it is a growth, extending in many cases through centuries. This growth is to be traced in the main through a series of cases; and much the shortest and best, if not the only way of mastering the doctrine effectually is by studying the cases in which it is embodied. But the cases which are useful and necessary for this purpose at the present day bear an exceedingly small proportion to all that have been reported. The vast majority are useless and worse than useless for any purpose of systematic study. Moreover, the number of fundamental legal doctrines is much less than is commonly supposed; the many different guises in which the same doctrine is constantly making its appearance, and the great extent to which legal treatises are a repetition of each other, being the cause of much misapprehension. If these doctrines could be so classified and arranged that each should be found in its proper place, and nowhere else, they would cease to be formidable from their number. It seemed to me, therefore, to be possible to take such a branch of the law as Contracts, for example, and, without exceeding comparatively moderate limits, to select, classify, and arrange all the cases which had contributed in any important degree to the growth, development, or establishment of any of its essential doctrines; and that such a work could not fail to be of material service to all who desire to study that branch of law systematically and in its original sources.
  It is upon this principle that the present volume has been prepared. It begins the subject of Contracts, and embraces the important topics of Mutual Consent, Consideration, and Conditional Contracts. Though complete in itself, it is my expectation that it will be followed by other volumes upon the same plan; but I have as yet formed go definite opinion as to how far the design will be carried. A volume upon Sales of Personal Property is more than half completed, and will be published within a few months.
   CAMBRIDGE, Oct. 1, 1871.