【正确答案】 A

【答案解析】[理由] 原句有对等连接词“not...but...”(并非……而是……)连接两对等的名词an easy book(一本简单的书)和one(一本书，等于a book)，故知空格后应置关系代词that(=which)，以引导定语从句，修饰先行词one。原句可改写如下：
The handbook is not an easy book to read, but a book that is very useful to anyone who wants to understand the inner workings of this company.
根据上述，(A)应为正选。