A "preview copy" is a version of something, like a book or a movie, that is shown or given to people before the final version is released. It allows people to get a sneak peek or an early look at the content. Full definition
I'm genuinely thrilled to be plotting out my next few years of self - published book releases, working on the platform, sharing preview copies with fans, and getting feedback. (alanrinzler.com)
The best way to generate buzz about your book is to give out preview copies before the release date. (nonfictionauthorsassociation.com)
They also had a developer preview copy of Windows 8 which really made this model a step in the right direction. (goodereader.com)