From the back cover:

The DC Vault unlocks DC Comics' most fascinating secrets and deeply buried treasures, presenting a colorful array of historic and never-before-published memorabilia, including early sketches, covers, memos, press materials, and much more. From a working reproduction of a 1942 Junior Justice Society of America decoder, to the original pencils and inks for a Wonder Woman comic book, this dazzling chronicle contains more than 25 plastic-encased archival pieces for readers to pull out and examine -- all while learning about the artists, writers, and world-famous superheroes that make up the DC universe.