Hornbooks are valuable assets to law students. The usual practice at Law School is to use a Casebook, which contains summations of major cases. The student then "briefs" that case by summing up the major points of law used to determine the ruling in that particular case. Of course, that presupposes that the student knows what the principle points of law are in that case. And Law Schools DO NOT teach that: it is up to the student to acquire the neccessary understanding.
That is where the Hornbook comes in. Unlike the Case Books, the Hornbooks EXPLAIN the law. By studying the Hornbook first, one has a better chance of understanding the law as it is applied in real life.

