《恐怖谷》为长篇小说，是系列第七部。叙述的是伯尔斯通庄园主人道格拉斯不幸惨遭杀害，尸体旁边留有卡片，上面潦草地写着用数字和字母组成的字样。道格拉斯临终前和妻子提起一个叫"恐怖谷"的地方，说自己身陷"恐怖谷"，无法逃避这场灾难。福尔摩斯解开了密码，却也卷入一场杀人案中，几经波折，福尔摩斯最终抓获了隐藏在黑暗中掌握众人命运的黑手。小说中所写的平克顿侦探卧底的斗争方法，后来很多惊险作品都有借鉴模仿，即所谓"打入敌人心脏"的战法。(The Valley of Fear is a novel and the seventh division of the series. It told a story that Mr. Douglas the master of Birlstone Manor House was killed. A card illegibly indicated with words consisting of figures and letters was left by his dead body. Douglas told his wife before his death a place named "Valley of Fear". He said he was trapped there and could not escape from the disaster. Holmes decoded the password but was involved in a murder case. After several twists and turns, Holmes captured the black hand controlling others' fate in darkness. The way to fight of Detective Pinkerton with undercover was mostly mimicked by many adventurous works. That is the tactics of "infiltrating into the enemy's heart".)