The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A book containing the rudiments of any science or branch
      of knowledge; a manual; a handbook.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hornbook
    n 1: a primer that provides instruction in the rudiments or
         basic skills of a branch of knowledge

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "hornbook":
   abecedarium, abecedary, alphabet, alphabet book, and arithmetic,
   basics, battledore, casebook, elementary education, elements,
   exercise book, first principles, first steps, gradus, grammar,
   induction, initiation, introduction, manual, manual of instruction,
   outlines, primer, principia, principles, propaedeutic, reader,
   reading, rudiments, schoolbook, speller, spelling book, t, text,
   workbook, writing