You keep it to translate the symbols for the underground door.
(see related question)
You have to collect all the pieces of the missing page of the journal.
There is no journal on Nabooti Island. But there is a ripped part of a journal that you need to find on Shark Tooth island.
open its on the missing piece of the journal
It is on top of the ruins. Push the big block to the right, then climb the vine and jump left from the wooden platform.
It's on top of the ruins
Professor Hammerhead has become the victim of the giant shark Booga, which he was studying as part of the island legends. He is trapped on an island in Booga Bay. His journal is in the Shark Museum, but it is missing a page. The page is on top of the ruins, and reveals the meaning of the ancient hieroglyphic symbols.
Shark Tooth Island - Professor Hammerhead's Journal is in the Shark Museum. But the important missing page is on the roof of the ruins.Time Tangled Island - Leonardo Da Vinci's notebook is on a ledge at the Statue of Liberty (1882 AD, 10 o'clock on the time device)Astro Knights Island - the journal is in Mordred's underground lab, and the missing page is under Mordred's bed in the Museum(see related questions)
Take the blimp to the island. Get the coconut milk from the guy on Main Street and continue right to the Shark Museum, where you can read the journal of Professor Hammerhead. His missing journal page is on top of the ruins farther right. Entering the ruins, you can collect the ingredients for a sleeping potion. You need it to get past the shark on Booga Bay (far right of the island).
Swim right to the island to rescue the missing boy. Lead him back to the beach.
Visit the Shark Museum and read the journal there. You have to collect three ingredients and take them to the medicine man to create a sleeping potion. You have to put Booga the shark to sleep before you can swim out to the island where the missing boy is. (see related question)
The journal is in the Museum but is missing a page. The page is on top of the Ancient Ruins. To get it, push the big rock until you can jump to the vines. Climb to the platform and jump hard left.
The missing page is on top of the ruins, and shows the translation you need to OPEN the door in the ruins. (Push the rock over to reach the vines, climb and jump to the roof.)