If you snagged a new Nexus 9 this past week, hit up the Google Play Books app. It looks like Google is offering a free book from a selection of best-sellers, and all you have to do is accept. We don't yet know if other devices are eligible or if there are geographic restrictions, but you might as well check.
There should be a big banner at the top of the app if you're eligible. It might also pop up on the Google Play widget. So far we've only seen it on the Nexus 9 in the US. Google offers titles like Gone by Michael Grant and Shake by Carli Davidson. You can only pick one book from the list, which will then be added to your library.
We've reached out to Google in hopes of finding out if this is only for the Nexus 9, or if other devices can get a freebie too. We'll update if Google gets back to us.