If you have a saved book collection for the old book then simply use the clear layout button.
You can either drag images from the filmstrip into the blank pages or use auto-flow.
Save as a new book collection.
I am seeing the same problem!
Try first placing the selected photos in a standard Collection, select these and open the Book module.
I found that a 'new' collection gave the 'old' photos, but if I then selected another collection I only saw the photos in that collection.
My only suggestion is to click the [Clear Book] tab above the main window, or click the [Clear Layout] button in the Auto Layout panel and you may then only see the selected photos.
I do not know if the bug has been reported at- Lightroom Classic CC | Photoshop Family Customer Community