答案：B

这道题容易误选C. that，因为当先行词被形容词最高级或the first和the last修饰时定语从句的关系词用that而不用which，一看见有the first (book)就匆忙选that。其实从句的先行词是three books而不是the first (book), the first (book)是从句的内容。这是个非限制定语从句，所以应选B. of which， the first of which="the" first of three books。