I am missing photos out of an album, how do I recover?

are you talking about a photo book? In the projects section? (My Projects)

Or do you mean an album where you just copy images into it. In the albums section. (My Albums)

I suspect you mean a photo book - there is not much work in creating an album. If so, which book creation app were you using. The apple one is no longer supported, since apple no longer offer printing services.

If a third party app, you might need to contact the developer of the app.

Are the photos only missing from the book, or from the whole library? If from the whole library, try looking in recently deleted folder.

Alternatively - if you use time machine and it has taken a backup of the library between doing the work, and the photos going missing then you may be able to recover it that way.

Are the photos still in the All Photos album, or are they gone everywhere?

Did you delete the photos from another album, for example Imports, after adding them to your album?

Albums do not store copies of photos. All photos are stored only once in the library, independent of the number too albums they have been added to. The albums only reference the photos. Deleting a photo anywhere in your library will delete it from all albums and projects, that have been using the photo. The same goes for deleting photos from other devices, if you are using iCloud Photos and all devices are syncing the Photos Library with iCloud. Deleting a photo will delete it from iCloud Photos and all synced devices as well.

If you have deleted the photos after adding them to your album, and the photos are also gone from "recently Deleted", you have to use your backups to recover the photos. Are you making regular backups, for example with Time Machine?

Sometimes photos are apparently missing, if Photos opened a different library than before, for example, if your library is on an external volume, and the volume is no longer connected. You can see, which Photos Library is currently open in Photos, if you open the Photos > Preferences > General tab. At the top you can see a button "Show in Finder". Is Photos using the correct library? If not, make sure all external drives are plugged in, then quit Photos and launch it again while holding down the options key ⌥. Release the key, when the library chooser dialog appears, then select a different library.