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I select photos, open the book module, then immediately "Create a Saved Book" Collection.
I always re-open the book by selecting this 'Book' collection, or clicking on the small white arrow at the end of the Book collections name.
As to the question- To add more photos- Drag them from the Library Grid View onto the 'Saved Book' collection in the collections panel.
The 'added' photos will appear at the right-end of the filmstrip and can be dragged onto pages in the book.
In addition to what wobertc says you can go to the collections panel and right-click on the saved book collection and choose “Set As Target Collection”
You can then randomly select photos from anywhere in the grid e.g. folders, collections, quick collection and simply press the B key to add photos instead of drag & drop.