The Collaborative International Dictionary of English v.0.48:

Liber \Li"ber\ (l[imac]"b[~e]r), n. [L. See Libel.] (Bot.)
   The inner bark of plants, lying next to the wood. It usually
   contains a large proportion of woody, fibrous cells, and is,
   therefore, the part from which the fiber of the plant is
   obtained, as that of hemp, etc.
   [1913 Webster]

   Liber cells, elongated woody cells found in the liber.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

LIBER. A book; a principal subdivision of a literary work: thus, the
Pandects, or Digest of the Civil Law, is divided into fifty books.