of them. Cards that are included in them will or will not help an agent get all 39 Clues, but by getting the cards inside of them, you may complete a Card Combo and receive a Clue. The cards that are in them are random, and so it is possible to get doubles or more. However in the Cahills vs. Vespers Series, the card packs have sixteen set cards.
Each card pack goes with certain books, so the first one goes with The Maze of Bones, One False Note, and The Sword Thief. Each pack also include a small booklet that includes information on characters, and various codes and ciphers.