A "complimentary copy" refers to a free or gift version of something, usually a book or magazine, given to someone for their benefit or enjoyment without having to pay for it. Full definition
Receive a few complimentary copies of your book in the mail (and purchase more at your author's discount price). (kathyide.com)
You will share your views with readers, receive complimentary copies, gain credibility, and may even be invited by a publication to write reviews and articles. (twilighttimesbooks.com)
About used books - if you were given complimentary copies and you gave them out, they can sell those again. (accrispin.blogspot.ru)