No iBook app on iPhone X; cannot find app in App Store
Open this link on your iOS device, then install the app.

 iBooks by Apple on the App Store

When searching for built in apps on the App Store, ‘make sure that you use the exact name of the app’ per the Apple KB article Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch. The exact name is ‘iBooks’ so search for this word on the App Store and you should find the app. The first item may be an ad, but the first result below any ad should be the right app.