[误] To find a book we want in so many books is just as difficult as looking for a needle in the sea.

[正] To find a book we want in so many books is just as difficult as looking for a needle in a haystack.

注：a needle in a haystack(干草堆里的一根针)确实难找，与我们“大海捞针”的难度应该是半斤八两，而且，

它可是英语中约定俗成的用法啊!