推荐一套学习英语词汇的好教材

学习英语词汇的经典教材，Cambridge Press出品，有大量的例句，还有课后练习，深入浅出，淳淳善导。

共为三本书，分低中高三级水平，每本在Amazon上都卖20多刀(e.g. http://www.amazon.com/English-Vocabulary-Advanced-Michael-McCarthy/dp/0521653975)

现有pdf版下载

English Vocabulary in Use: Elementary.
English Vocabulary in Use: Pre-Intermediate to Intermediate.
English Vocabulary in Use: Advance