They're called "page proofs," and I recently received a whole bundle of them from my publisher. Once upon a time, page proofs came in a cardboard box that landed on my doorstep; now, I get an email, directing me to where I can download the images from an FTP server. They represent the last chance to correct any errors or typos in a book being readied for publication.
So if they're "proofs," what do they prove, exactly? In this case, they represent pretty compelling evidence that my book on Martha Raye should be out shortly. It's available for pre-order on Amazon and elsewhere. I hope you'll check it out.