In the card left for Kady at Fuzz Beat: 3 symbols that stand for Aquarius, Gemini and Earth (AGE); a latitude longitude, in two different formats which points to the backyard of a house in Farnham, England; 793.8 which in the Dewey Decimal System is the section of "Magic and related activities" in a library (1914 is probably the year of publication of a specific book being referenced in a coded way by the preceding numbers starting with the symbol for pi).
The company Penny and Kady go to for the overdue book is called FuzzBeat, which specializes in "serious news stories and cat videos". It's a spoof of the company BuzzFeed.
At 25:12 when Harriet shows Kady and Penny what is really on the computer screen the image shown is similar to "the tree of life" from the Kaballah which is a part of Jewish mysticism.