He finally tracked down the book the he wanted.他最终找到了他想要的那本书

你的原句有点须改正的地方
He finally tracked down the book( that/ which) he wanted.其中
that/which he wanted 为定语从句,修饰 the book(先行词）,由于先行词为物,故用that/ which作为定语从句的引导词.注意：that/ which本身同时作为定语从句的主语或宾语（表语）,所以定语从句里没the book.（注：如引导词作为定语从句的宾语或表语,则引导词可省略）
祝你进步!理解请采纳!