Features more than one thousand primary entries, along with more than two thousand examples of questionable style and the misuse of language, providing valuable lessons for students, writers, and speakers.
It's a dictionary of usage. Depending on how in depth they are, it's seldom that you'll find information in one that's not in all of them (of comparable depth of course). If you do find new information in one, and the source isn't reputable, then perhaps you should be skeptical and do some research.
*Excellent* usage guide - I've always found the answer I was looking for when I turn to this source. It ranks ahead of the Chicago Manual & Strunk and White for me.