Adding more pictures to a photobook in progress.

I am nearly finished making a photobook, but now I need to add an extra picture. I import it to the library and it sits there, I can use Develop on it,  but I cannot get it to appear in the photobook - even if I designate it a cover page in the Library!

Deleting is easy but how do I add an extra picture to a photobook in progress? 

There are hours of work involved already, so I don't want to just clear the book and start again as each page always needs a lot of manual adjustment and captioning!