The clue is an almost-complete set of covers of the Tintin series of comics (or "graphic novels" if you prefer). As the blank rectangle with the question mark implies, one is missing. Most (all?) teams had at least one member who recognized Tintin, and because the island had good cell coverage it was possible to call a friend to go to Amazon or the innumerable Tintin fan websites, describe the covers to them, and by process of elimination find the missing book.

In left-to-right, top-to-bottom order, the titles of the American versions of the books are:

1. Tintin in America
2. The Castafiore Emerald
3. Cigars of the Pharaoh
4. The Red Sea Sharks
5. The Shooting Star
6. Destination Moon
7. Tintin in the Congo
8. The Crab With the Golden Claws
9. The Secret of the Unicorn
10. The Blue Lotus
11. Red Rackham's Treasure
12. Land of Black Gold
13. The Broken Ear
14. Tintin and the Picaros
15. King Ottokar's Sceptre
16. The Seven Crystal Balls
17. The Adventures of Tintin in the Land of the Soviets
18. Prisoners of the Sun
19. Tintin in Tibet
20. The Calculus Affair
21. Flight 714
22. Explorers on the Moon

There is another book called Tintin and Alph-art, which was unfinished by the author but published a few years ago in sketchbook form. This is not considered to be a "full-fledged" Tintin book. There are also adaptations of Tintin movies in comic form, and those don't count either. The only remaining book is The Black Island, and indeed the next location is Black Island on Lake Mead.

Clue author: Walter Smith