a page bearing a religious text or the alphabet, held in a frame with a thin window of flattened cattle horn over it
2.
any elementary primer
hornbook in American English
(ˈhɔrnˌbʊk)
noun
1.
a sheet of parchment with the alphabet, a table of numbers, etc. on it, mounted on a small board with a handle and protected by a thin, transparent plate of horn: it was formerly used as a child's primer