USGPO Style Manual

The U.S. Government Printing Office Style Manual offers free, on-line advice on capitalization, spelling, punctuation, and many other areas of style. Its About page says, "The Style Manual is the product of many years of public printing experience, and its rules are based on principles of good usage and custom in the printing trade." The on-line version allows you to browse the table of contents or search for specific words or topics.