Wordorigins.org

Webster’s Dictionary of English Usage; Merriam Webster; 1989.

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.