How to install iBooks on iPad
The simplest way might be to open your web browser on the iPad and open a link to a book.
That will take you to the store if you need the app and take you to the app if you just lost where the icon is installed.
https://itunes.apple.com/us/book/apple-watch-user-guide/id985786488
If that doesn't work, your iOS is a bit messed up and backing up your data and restoring the device would be the next step. More info at https://help.apple.com/ipad/11/