When you are finally ready with the design you need to print the book. It's a very important thing because you will, again, learn a lot of new information. To give you one example, if you know that when you choose black background from the options, it's not totally black, so of you place true black square on this background, you will be able to see the borders. Imagine if you didn't know that and printed a hundred books with this mistake!
Also you need to see the actual paper you will use, for example to see how reflective it is. Imagine you view a book sitting under a ceiling light. With paper that's too glossy the reflection of light will distort the image and blind you.
Next consideration is fingerprints and scratches. Avoid covers materials that scratch easily (like black paper) or collect dust (certain types of linen). See if the book can have protecting box to avoid bending corners. This about full or partial dust jacket.
Another important consideration is if you want a lay-flat or not. Lay-flat had very thick pages and lays flat when open, so it doesn't have a gutter. It's a really good option for a coffee-table book as well as the book with whole spread pictures, but it's also quite expansive and heavy option.
Finally, remember that you book will be printed with ink, so for example if you have a white cursive font on black background, it can become less readable because in will spread a bit into it. If you have a lot of text, this might be important to consider.
When you have a printed book in your hands, go ahead and do more of viewer interviews. This time look at person's actions more. Does he/she hold the book in one hand or both hands or on their lap? Is it easy to flip a page? Does flipping leave fingerprints? What parts of the book person flip through, what parts do they look at and do they read all text? any text? Do they flip pages back and forward of only one direction? All this will tell you how the person experiences your story and how engaged they are.
(to be continued) https://alionakuznetsova.com/photobook-tutorial-publishing/