Returning to this game months later, I am finally able to solve 1.10. The easiest way to explain it is do not read it as a multiplication problem. You are just trying to get 13 digits which, if read as intended, you will get.
A hint for 1.10? lack of parenthesis and such make it hard to interpret. I know what I am supposed to do but (SPOILER WARNING-ISH?) no number I get is a valid ISBN code. I'm aware this issue is being worked on, especially for the steam release but a little clarification would go a long way
 "Mystery Puzzle Hunt : A Realistic Riddle Game"){kind=icon}