The Collaborative International Dictionary of English v.0.48:

Compendium \Com*pen"di*um\, n.; pl. E. Compendiums, L.
   Compendia. [L. compendium that which is weighed, saved, or
   shortened, a short way, fr. compendere to weigh; com- +
   pendere to weigh. See Pension, and cf. Compend.]
   A brief compilation or composition, containing the principal
   heads, or general principles, of a larger work or system; an
   abridgment; an epitome; a compend; a condensed summary.
   [1913 Webster]

         A short system or compendium of a science. --I. Watts.

   Syn: See Abridgment.
        [1913 Webster]

WordNet (r) 3.0 (2006):

compendium
    n 1: a publication containing a variety of works [syn:
         collection, compendium]
    2: a concise but comprehensive summary of a larger work

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "compendium":
   Baedeker, abbreviation, abbreviature, abrege, abridgment, abstract,
   brief, capsule, compend, condensation, condensed version,
   conspectus, digest, draft, enchiridion, epitome, guide, guidebook,
   head, manual, outline, overview, pandect, precis, review, rubric,
   shortened version, skeleton, sketch, survey, syllabus, synopsis,
   thumbnail sketch, topical outline, vade mecum