Open the Photos tab on the left and click Add Photos. You can choose up to 1,000 photos at a time from your computer (phone photos, HEIC included) or import an album from Google Photos. A progress bar shows them arriving; duplicates are skipped.
Then either run Auto-Arrange to build the book from everything in the library, or drag photos onto pages yourself. Photos that are not yet on any page show under the Unused filter.
Blurry at print size? A small warning on a photo means it is too low-resolution for the page it is on — use it smaller, or swap in the original from your phone or camera.
Tap Help at the bottom right of any page and tell us what is happening. Your message comes straight to us and we usually reply within a few hours.