How do I get my photos from iCloud?

Open the Photos app on your Mac

Since your photos are in iCloud, you can export them straight from your Mac.

Select your pictures and export the originals

Pick the photos you want from your library or a private album. Go to File → Export → Export Unmodified Originals and save them to a folder. This can take a while, so grab a drink.

You're all set

Now all you need to do is create your book and drag the folder in.