Source : Webster's Revised Unabridged Dictionary (1913)

Hornbook \Horn"book`\, n.
   1. 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. ``He teaches boys the hornbook.''
      --Shak.

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