I've just started my first large book project in LR4 and can't believe how clunky adding images to an existing project needs to be.
I would have thought that the most intuitive way to add extra images to an existing book would be to select a collection that contains the image then drag the image onto the page and it would just be added to the book, but no. As soon as you select a different collection you go away from the saved book and it opens a new project.
So to add some more images to a book it seems you have to go back to the library, choose the image(s) then drag them to the correct book collection in the library, then go back to the book module, try to find the new image in the film strip, then drag it onto the page.
Am I missing something ?