primer vs hornbook

primer and hornbook both are nouns.

Word	Noun	Adjective	Verb	Adverb
primer	Yes	No	No	No
hornbook	Yes	No	No	No

As nouns, hornbook is a hyponym of primer; that is, hornbook is a word with a more specific, narrower meaning than primer:

primer: an introductory textbook
hornbook: a primer that provides instruction in the rudiments or basic skills of a branch of knowledge

Other hyponyms of primer include speller.

primer (noun)	hornbook (noun)
any igniter that is used to initiate the burning of a propellant	a primer that provides instruction in the rudiments or basic skills of a branch of knowledge
the first or preliminary coat of paint or size applied to a surface
an introductory textbook