Yes, custom would do if I had thought of it at the start. I duplicated the book, set the page sizes as I now know they should be using custom and it totally wreaks havoc with the book layout and loses the text entirely. I know that I could painstakingly remake the book but, needless to say, I much rather find a way to adjust the page size while retaining the current layout.
Redo the book. This is the only way you will be satisfied. IME, though "painstaking", re-creating existing work goes very quickly (you don't have to make any decisions -- it's work without thought).
A publisher might be able to print your book with the pages centered on whatever size stock they use.
You can resize PDFs. Google that or "scale PDF". The problem here is that resizing is sure to reduce the quality of the final print. How much? How much is OK?