Please do not post solutions to puzzles, but rather post hints as appropriate in response to queries
Type [spoiler] ... [/spoiler] around the outside of any hints so they can't be read by accident - e.g. [spoiler]have you tried looking at the corner of the page?[/spoiler]
I'm wondering about the small scrap of paper with letters
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=V (voltage, related to "Potential Difference"?) I R (current and resistance) I^2R (power) It seems like there should be a way to get a hidden code from that, but I can't figure it out. Is there more that was torn off the bottom?
The shape and tear matches the scrap with code 31, which is the same but larger. FYI, code 31 has some binary: 1001010110110101010 and 0101001011010101101.
Along those same lines...
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The bottom coffee ring stain with code 40 matches the bottom one with code 56
and...
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The random characters and horizontal bar on the cover are reminiscent of the same theme with code 1. But that could easily be just artistic design.