Collectors want prime dust jackets. Do if you're grading a book, look at the jacket first, and then move on to the book itself. When you start looking at catalogs, you'll see most dealers will give one grade for the book itself, another for the dust jacket. The whole package is important, but the dust jacket has priority. Again, without exception for books produced in the last twenty-five years, there is nothing more important to the book's value than the dust jacket.
Most limited editions come sans dust wrapper.
Watch for them. They're usually obvious, but looking for them can make it worth your while to pull an unjacketed book or two off the shelves while you're browsing. Very Fine, Fine, Near Fine, Very Good, Good, Poor. A Very Fine book is a book in perfect condition. There is virtually no sign that this book has ever been read.