When Grandmama was four years old
She learnt to say her A.B.C.
And went to school like you and me...
But in poor grannie's book
Was only one page--look!
And in case this should be torn
It was covered up with horn
And nailed upon a wooden back
With bits of brass and many a tack.
So remember when you look
At the pictures in your book
That poor Grannie long ago
Had but a HORNBOOK as you know
But learnt to read to write to rhyme
And so will you my dears in time.

Hornbooks
Hornbooks were most often made of oak, the letters were covered with transparent horn and the whole fastened down with brass strips and tacks. Sometimes leather was used to cover the wood or hold down the letters. Wealthy children had hornbooks made of ivory or silver. Schools used hornbooks made of brass or lead. The text consisted of the alphabet or the alphabet and numbers, or all of the above with the Lord's Prayer.
The hornbook originated in England around 1450 and is mentioned in Shakespeare's Love's Labour's Lost, act 5, scene 1, where the ba, the a, e, i, o, u, and the horn, are alluded to by Moth:
ARMADO. [To HOLOFERNES] Monsieur, are you not lett'red?
MOTH. Yes, he teaches boys the hornbook. What is a, b, spelt backward with the horn on his head?
HOLOFERNES. Ba, pueritia, with a horn added.
MOTH. Ba, most silly sheep with a horn. You hear his learning.
HOLOFERNES. Quis, quis, thou consonant?
MOTH. The third of the five vowels, if You repeat them; or the fifth, if I.
HOLOFERNES. I will repeat them: a, e, I-
MOTH. The sheep; the other two concludes it: o, U.
It is also described by Ben Jonson in his play, Volpone, act 4, scene 2:
CORVINO: ... And yet I hope that I may say, these eyes
Have seen her glued unto that piece of cedar,
That fine well-timber'd gallant; and that here
The letters may be read, through the horn,
That make the story perfect.
Use of the hornbook was especially prevalent in the New England colonies.

A picture of a "chapman," who sold hornbooks and chapbooks. (From a drawing first published in Rome in 1646.)