You can create a PDF file of your photo book to view in iBooks by creating a PDF preview.
And iBook Author can import Pages documents, but I do not know of any way to export directly a photo book created in Photos to iBook Author.
A quick-and-dirty way would be to save the PDP preview as separate photos and then to use a photo of each page as a page in your iBook. That would be very wasteful, however and you would not be able to make the photos interactive. You will get much better results, if you start over in iBook Author and import the photos in an optimized size for iBooks to save storage and copy and paste the captions from your photo book individually into the iBook.
To create a PDF preview see: https://help.apple.com/photos/mac/1.0/?lang=en#/pht6e15caf3
- Click Projects in the toolbar, then double-click the project you want to save as a PDF.
- Do one of the following:
- Choose File > Print. Click the PDF pop-up menu at the bottom of the dialog and choose Save as PDF.
- Press the Option key while you click the Buy button.
- Press the Control key while you click in the background of your project, then choose “Save [project] as PDF.”
- Enter a name and select a location, then click Save.
When you open the PDF file in Preview and reveal the sidebar with thumbnails of the pages, you can drag individual pages to the Desktop to save them as a one-page PDF file. Import these to iBook Author.