Briar's Book (Japanese: ゼロの秘宝 Hidden Treasure of Zero) is a Key Item introduced in Generation IX. It is a copy of Briar's book about her and the player's expedition to the Area Zero Underdepths, titled The Hidden Treasure of Area Zero.
A proof copy of the exploration notes covering the Area Zero Underdepths. It's filled with long paragraphs describing what happened, along with Briar's thoughts.