It is when you are using what you have learned from books that you wish you had read more.

解释 ( paraphrase )
读书到了要用的时候才悔恨读得太少。
这条谚语告诉我们：读书不多，到应用的时候就会感到知识不足。所以应该多读书，多积累知识。

This proverb reminds us that we can never read widely enough.

方：才。
      Only, just when.

造句 ( example )
书到用时方恨少，什么都应该赶早学习，否则到了关键时候就会觉得后悔。