A hornbook is a book that serves as primer

Primary education

A primary school is an institution in which children receive the first stage of compulsory education known as primary or elementary education. Primary school is the preferred term in the United Kingdom and many Commonwealth Nations, and in most publications of the United Nations Educational,...

 for study. The hornbook originated in England in 1450 (Huey, Edmund Burke). The term has been applied to a few different study materials in different fields. In children's education, in the years before modern education materials were used, it referred to a leaf or page containing the alphabet, religious materials, etc., covered with a sheet of transparent horn (or mica) and fixed in a frame with a handle.
In United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 law

Law

Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

, a hornbook is a text that gives an overview of a particular area of law.