Leonardo da Vinci's Notebook can be found in the 1882 AD time period of Time Tangled Island.

To get it, go to the very left, where you will see a man next to a box and a barrel. Jump onto the box, then to the roof of the house by the platforms marked “Gadget, Gauthier et Cie”. Move along the roof, and jump onto the chimney. Now jump onto the brown statue on the right. Jump up to the wooden ledge above, then another ledge. Then jump to the left with all your might, and you should land on a ledge with the Notebook.

When you return it to Leonardo da Vinci in his workshop in 1516 AD, that time period will be restored, and he will award you with some wings.