late 14c., "prayer-book," also "school book" (senses not distinguished in Middle Ages, as reading was taught from prayer books), from M.L. primarius,
from L. primus
"first" (see prime
(adj.)). The exact sense is uncertain, and the word may be from prime (n.)
on the same notion as a "Book of Hours." Meaning "small introductory book on any topic" is from 1807.