Grammar Girl has a knack for putting the most useful information from dictionaries and style guides together into one fun-to-read place. I teach English and would love to require (or give) this book to each student (not to mention most of my friends and family members).
Even if you think you have a firm grasp on easily confused words (capital/capitol, hone/home, trooper/trouper), you might find that you're wrong. Grammar Girl gives enough background on the words to help you understand why the words are often confused and then even offers tips for how to prevent mixing them up again. (Take "trouper," for example. So many people spell it "trooper" when they mean "trouper," and Grammar Girl explains that the expression "What a trouper" comes from when an actor [a member of a troupe] nails a particularly tough role. That is why it should be "trouper," not "trooper" in that context. Neat, huh?)
Keep in mind that you can't count on spell and grammar check to catch these errors in usage, especially if the words mixed up are the same part of speech, so this small, handy book belongs in every writer's desk and every student's backpack. Parents, use this (and Grammar Girl's other Quick and Dirty Tips books, such as Grammar Girl's 101 Words Every High School Graduate Needs to Know) to quiz your kids at the dinner table. These books are small, inexpensive, fun, useful, and portable.