Wordorigins.org

Merriam-Webster’s Dictionary of English Usage. Springfield, Mass.: Merriam-Webster, 1994.

If you can only afford one usage manual, this is the one to get; far and away the best on the market. It provides a detailed history of the development of the usages it presents. The only problem is that the wealth of information and background it provides makes it sometimes difficult to use as a quick reference; there is simply so much that it just sucks you in.