Q: Do the book tokens need to be on the solution card when the aspect is solved?
A: No. Book tokens on any cards in the center row while solving an aspect are considered returned and set aside (with the solution card) to be counted at the end of the case for scoring purposes.
Q: If a book token matches the aspect of the book-related card, does that card count as 2 cards of that aspect?
A: No. A book token on a clue card makes that card behave as if it possesses both aspect types, but does not make it act like 2 separate cards. If a clue card contains a book token of the same aspect type, it only counts as 1 clue card of that type when solving aspects, 1 card when gaining ideas from a shift focus action, and the suspect pawn ending its movement at that card will only trigger the corresponding event once.
Q: Is it possible to have a negative score at the end of this case?
A: Though highly unlikely, it is technically possible to have a negative score at the end of this case if you did not solve any aspects before the case ended and thus were unable to return any library books, in which case your score would be -3.