The Red Ledger
Sam: Well, it describes the horrible experiments performed on the camp's population. Magical experiments.
Golem: More horrible than words.
The Red Ledger is a valuable item which belonged to the Thule Society -- a log book from a Nazi Compound in Belarus which contains the information on multiple experiments involving Necromancy as well as the names of every dead Thule member who was reanimated. The Red Ledger was in the possession of the creator of the experiments involving the reanimation of dead Thule Members, Commandant Eckhart, until it was lost in a fire on a Nazi compound in Vitsyebsk, Belarus, in 1944. Rabbi Issac Bass was convinced that it survived the fire and located it in a college library in Wilkes-Barre, Pennsylvania. Sam and Dean catch up with Issac's grandson and retrieve the Red Ledger after a run-in with Eckhart.
It is currently in the possession of Aaron Bass and the Golem of Vitsyebsk, who are using it to hunt the Thule to extinction. By late 2016, they had used the information contained within to kill half of the Thule's entire roster. The Men of Letters Bunker also contains a copy of the information in the Red Ledger, updated by Sam after it was recovered from the library and catalogued upon his return to the Bunker.
Rabbi Bass goes into a college library in Pennsylvania where he locates the Red Ledger as Manuscript: FD113 from the Holtzinger Estate. Before he is killed by Torvald, Rabbi Bass leaves his grandson, Aaron Bass, a message with a call number: QL673W38. Sam is able to locate the Red Ledger, which Issac Bass switched with The Explorer's Guide to North American Birds, but is magically poisoned by Torvald in the process. The Golem is able to snap Torvald's neck before the poison can take full effect.
Sam translates the Red Ledger and discovers that it describes experiments the Thule Society conducted up to 1944 as well as containing the names of every dead Thule member who was reanimated. Eckhart and his men come to locate and retrieve the Red Ledger, but they discover that Torvald was killed by the Golem. They use the Golem's clay to locate the Winchesters and Aaron, and Eckhart is able to shut down the Golem and remove the scroll from his mouth. They locate the Red Ledger and Eckhart is about to kill Bass and the Winchesters, but Aaron attacks him. Sam and Dean are able to kill two of Eckhart's men, but one escapes. Eckhart threatens them by stating that they will never kill all the Thule, but Sam and Dean kill him nonetheless.
The Winchesters decide to leave the Red Ledger with Aaron, who is finally ready to take charge of the Golem and carry on his grandfather's legacy by eliminating the Thule. Sam makes copies of the Thule's Red Ledger for the Men of Letters Bunker and makes a reference card to update the library.
Sam and Dean call Aaron Bass when they discover that they are dealing with the Thule Society again. Aaron, who is currently in Berlin, Germany with the Golem, is tracking down the members of the Thule recorded in the Red Ledger. They've been able to kill half of the Thule members recorded in the ledger, most of which were killed by the Golem while Aaron has only killed six. He tells them that the Thule have mostly been sticking to the "Fatherland", but he was able to retrieve some of their documents on Das Blut --"The Blood"-- a mission in Columbus, Ohio, where all four members of the Thule High Command will all be in one place.