So is the book purchased from Google Play Books, or just the Nexus 7?
If the book was purchased from Google Play Books, then go to the My Library section of Google Play Books. Tap on the 3 vertical dot menu button associated with the book you want to delete (this can be a little tricky if you have big fingers--you need to tap the button to bring up the dialog box). If it's been temporarily downloaded to the device, you'll see a vertical pushpin icon in a blue circle at the lower right corner of the book entry; when you tap the 3 vertical dot menu button, you should see an option for "Don't Keep." This will remove the book from your local storage, but it will remain in your cloud library. If you want to remove it completely from your cloud library, tap the 3 vertical dot menu button again, and select "Delete from Library," as srkmagnus said above.