Although the card packs are not necessary to read the books, they are for the interactive website. The cards give you hints, clues and puzzles to the rest of the Cahill's and each of the branchs. The show you more of the branch members of the past and present. They have great artwork and information on them.
The only problem is that the cards are randomly inserted in each pack, and they have a rarity index. So for me I needed to buy around 15 packs to be able to complete the first series of cards. That is a pretty steep investment. If you are going to go full tilt and keep up to date on the Clues and participate at least weekly (more likely daily) on the 39 Clues website then this is a necessary investment to fully enjoy the experience.
As much as I enjoyed collecting the cards and going to the website, I found that as soon as I missed a week or two, things had changed and to catch up was not impossible, but a huge investment in time. Plus with over 50 cards per set and not including cards that were released though various stores and overseas stores, it was tough to keep up. ( )