How do i add additional images to a saved Blurb book within the Book Module

The Collection that you add the extra photos to must be a 'Saved Book Collection' that has the small 'book' icon. (and not a standard Collection!). Have you created a 'Saved Book Collection' ?

Then in Library Grid view  (Shortcut [G] ) you drag photos to the 'Saved Book Collection'.

You can even set this Book Collection to be the 'Target' Collection so that a simple keypress [B] will add selected photos to this collection from Library Grid view.

[Double-Click] the Book Collection to re-enter the Book module and your newly added photos will appear at the far end of the Film-strip, from there you can drag them onto the book pages.

Going forward, the 'Saved Book Collection' is the only collection that is relevant to your book design and any Standard Collection you might have started with can be ignored or deleted.