Creating a Print Ready Book in Design

You should insist on contacting the printer directly and finding out what their requirements are in terms of colours (RGB or CMYK images, spot colours, etc.), page size, image resolution, file formats, and more. Simply being told my someone in between you and the printer is like being told you have provided the wrong fuel for a vehicle without knowing what vehicle they are using.

I'm assuming your client is not the actual printery. So, the client must have received information from their printer. What was the detailed feedback? 

With so many potential issues, just saying something is not print ready is pretty meaningless. 

A print ready book follows the requirements of the printer. The place to start with a question like this is by requesting the precise layout requirements the printer is demanding.

The most basic rules for "print ready" are:

• That the pages are laid out to a standard size (usually 6x9 inches, 8.5x11 inches in US/North American trade);
• That the pages have adequate margins (usually 1 inch inside/spine, 1/2 inch or more outside/edge, and at least 1/2 inch top and bottom);
• That the margins alternate on right/left, recto/verso pages; right-hand pages need the inside margin on the left, left-hand pages, on the right;
• Usually, that the pages be numbered sequentially, except for short gaps around chapter beginnings.

All of this needs to be exported to a PDF with other characteristics defined by the printer, such as color space (even if the content is all black and white) and with or (usually) without page crop marks.

That's a start. But it sounds like you need to start with the very basics, and the first of those is to request print-ready specs from the printer. (Not any printer or something generic you might find on the web, but the actual printer who will be handling the job.)