In Photos is there a way to lock in album order for a book project?

no

you either have to add them one by one (or in small groups) or reorder in the book

tell Apple your needs for potential inclusion in a future version - http://www.apple.com/feedback/photos.html

LN

How do I keep the book from automatically sorting?

Try to create the book this way:

• First select all photos in your album with ⌘A .
• With all photos selected use the command "File > Create Book".

This way all photos will be added to the book pages in the order you had them in the album. The only drawback is, that you cannot prevent the auto fill of the pages this way.

The best way is really to do as Larry suggested: Drag the photos from the album to the book one by one, as you go. Keep the sidebar visible, and then you can drag directly from your album to the book in the sidebar. Add only the photos you will need for the next page, not all photos at once. That will save you a lot of scrolling through the unplaced photos.

You're welcome 🙂