I agree that instead of calling it "Hide" , they should just call it "Delete". For consistency, they use the same terminology for Music, Movies, and TV Shows as they do for Books. The actions are the same as for apps where deleting an app will remove it from the device, it can still be redownloaded by going to the App Store. In the Books app you can remove it from the phone to free up space on the device and it will still remain in the Library with the iCloud link. To remove it from the library, they call it "Hide". You can unhide them by going to your user profile where they will remain in case you want to download them again like you would for a purchased app in the App Store.
Info on Hiding and Unhiding Books is available here:
Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support
More info about the Library is available here:
Organize books in the Books app on iPhone - Apple Support