The Collaborative International Dictionary of English v.0.48:

hardback \hard"back`\, hardbacked \hard"backed`\, hardcover
\hard"cov*er\ hard-bound \hard"-bound`\adj.
   Having rigid front and back covers, usually boards covered
   with paper, cloth, or leather; -- of books. Contrasted with
   softcover and paperback.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

hardbound \hardbound\ adj.
   same as hardback; -- used of books.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

hardbound
    adj 1: having a hard back or cover; "hardback books" [syn:
           hardbacked, hardback, hardbound, hardcover]