What is a ''hornbook'' in The Witch of Blackbird Pond?
Since books were so expensive, a hornbook—basically a board with a handle--provided an alternative way for children to learn the basics as they began school. They were used in the 1400s-1700s in both England and America. A hornbook consisted of a board covered with a single page containing some basic information: perhaps the alphabet, a series of Roman numerals, or a short scripture. This "primer" was then covered with a thin piece of transparent horn to protect the paper from wear. As books became more affordable, the hornbook naturally became used less and less.
A hornbook was a thing that people used back then to help kids learn to read. It was a piece of wood that could have a page put on it. Then the page would be covered with a transparent sheet, usually made from a bit of horn. It is apparently possible to take a thin sheet of the horn of a cow and make a transparent sheet. This is how the hornbook got its name.
The purpose of a hornbook was to allow a kid to be able to look at a page without having to worry about getting it dirty or torn or otherwise ruined.