Nathan Drake uses a pocket-sized journal to document some of his various discoveries, along with other information. The journal also contains a few straightforward hints and tips that can aid him in solving some of the puzzles he encounters in his travels.
It contains a page for Nathan's various girlfriends and their addresses etc. Other than that, there are a few occasional doodles and photos of people he knows, such as two pages containing his drawings of "The Many Faces of Victor "Goddamn" Sullivan" and other goofy drawings like ¡SKELZOR!, a skeleton Nathan used to startle Sully and Chloe in Borneo and a cartoon drawing of his brother Sam in some broken stocks in Libertalia during the fourth game A Thief's End. It also holds a lot of info regarding the current discovery.
Along with more adventures come more pages, with information from previous exploits that aid Nathan in solving cryptic puzzles. Various references are also made, such as the "slippery naked guy" ranking below a "Demon Sasquatch" on the "scare-o-meter" (both of which outrank Sully's Mustache); or a sketch of an enraged statue which Drake has named "Angry Eddy" with the caption "I KILL YOU DRAKE!" The last page of Francis Drake's diary and Spanish coin that Nathan Drake found in the German U-boat are taped to the first and second pages of his journal respectively. There is also a drawing or picture of Tenzin's daughter Pema.