There is no
iBooks on my iPad. I went to the iTunes AppStore and searched for
iBooks and that showed anything and everything but that app.
So what is the correct way to use/install/download
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.
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/