Possibly, the boy William, instead of a horn-book, had an "A-B-C book," which often contained a catechism, in addition to the elementary reading matter. To this we have an allusion in King John, i. 1. 196:—
"Now your traveller—
He and his toothpick at my worship's mess,
And when my knightly stomach is sufficed,
Why, then I suck my teeth and catechise
My picked man of countries: 'My dear sir,'—
Thus, leaning on my elbow, I begin,—
'I shall beseech you'—that is question now;
And then comes answer like an Absey book."