The Collaborative International Dictionary of English v.0.48:

Instructive \In*struct"ive\, a. [Cf. F. instructif.]
   Conveying knowledge; serving to instruct or inform; as,
   experience furnishes very instructive lessons. --Addison.
   [1913 Webster]

         In various talk the instructive hours they past.
                                                  --Pope.
   -- In*struct"ive*ly, adv. -- In*struct"ive*ness, n.
   [1913 Webster]

         The pregnant instructiveness of the Scripture. --Boyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

instructive
    adj 1: serving to instruct or enlighten or inform [syn:
           instructive, informative] [ant: uninstructive]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "instructive":
   admonitory, advisory, authoritative, autodidactic, binding,
   canonical, cautionary, coeducational, commanding, communicative,
   compelling, consultative, consultatory, cultural, decretal,
   decretive, decretory, dictated, dictating, didactic, directive,
   disciplinary, edifying, educating, educational, educative,
   enlightening, exhortative, exhortatory, expostulative,
   expostulatory, formulary, hard and fast, helpful, homiletic,
   hortative, hortatory, illuminating, imperative, informational,
   informative, informing, initiatory, introductory, jussive,
   lecturing, mandating, mandatory, monitorial, monitory, moralistic,
   moralizing, obligating, official, peremptory, preaching, preachy,
   preceptive, prescribed, prescript, prescriptive, propaedeutic,
   recommendatory, regulation, remonstrant, remonstrative,
   remonstratory, revealing, rubric, self-teaching, sententious,
   standard, statutory, teaching, tuitionary, warning