WAY easy to overthink this puzzle. Below are some graded hints leading to the solution, which is a 4-digit code. It assumes you know key 107. If you don’t know it, then don’t continue. . .
Spoiler!
You only need both sides of page 111 to solve this puzzle (although looking at other pages in the book may help you out), and all of the text/artwork is irrelevant -- just the clue phrase and some observation.
Spoiler!
The clue phrase is an equation 'What is missing [from the page] X 30'. You are looking for a number to multiply to 30 to get the answer. That number is missing from the page.
Spoiler!
Compare the QR code page 111 with any other QR page in the book. There is a key difference between page 111 and the others--it is missing something that is usually there.
And the solution is below. DO NOT CLICK IF YOU DO NOT WANT TO KNOW THE ANSWER!!