(1):

(n.) The first book for children, or that from which in former times they learned their letters and rudiments; - so called because a sheet of horn covered the small, thin board of oak, or the slip of paper, on which the alphabet, digits, and often the Lord's Prayer, were written or printed; a primer.

(2):

(n.) A book containing the rudiments of any science or branch of knowledge; a manual; a handbook.