to serve as a reference; to provide useful information; to be helpful; to be informative; to be instructive
to refer to; to consult; to use as a reference
reference book; study-aid book; supplemental textbook; student handbook
invited witness; unsworn witness; person of interest; voluntary interviewee; expert advisor; consultative expert
reference data; reference material
for your reference; FYR; for your information; FYI
reference materials; specimen for reference; reference item; specimen
unofficial result; unrecognized result (esp. in sports)