In my business writing workshops, I give participants a list of resources, one of which is Common Errors in English. It is by Paul Brians' and is on the Washington State University website. I has hundreds of words people commonly mis-use, such as affect and effect; ensure, assure, and insure; then and than; that and which.
If you're not sure which word to use, check it out. I also have the book, which you can buy here:
William, James & Co.