Congratulations on your new iPhone XS! I understand you would like to know if there's a way to recover iBooks that disappeared after migrating to your new iPhone. I would be happy to help point you towards a resolution.
If the books were purchased from iBooks or Apple Books, follow these steps to redownload them to your iPhone XS:
Redownload apps, music, movies, TV shows, and books from the App Store, iTunes Store, and Apple Books - Apple Support
Specifically, check out this section:
Redownload books or audiobooks
You can redownload books in Apple Books on an iPhone, iPad, iPod touch, or Mac. You can redownload audiobooks in Apple Books on an iPhone, iPad, or iPod touch; or in iTunes on a Mac or PC.
On an iPhone, iPad, or iPod touch
- Open the Apple Books app.
- Tap or your photo in the upper-right corner of the screen.
- Depending on what you want to redownload, tap Books or Audiobooks under My Purchases.
- Tap "Not on this [device]," then tap All Books or All Audiobooks.
- Find the book or audiobook that you want to download, then tap . The item downloads to your device. Don't see the item that you purchased?
If the books were purchased and downloaded from another source, you would need to download them from that same source on your iPhone XS. If you've previously synced the missing books from your iPhone to iTunes, you can also sync the books to your new iPhone following these steps:
Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB - Apple Support
Take care!