Dust jackets increase a book's value tremendously. Dust jackets became popular in the 1920's with the purpose of dressing up a book and increasing the book's worth. Finding an older book with a relatively new dust cover still intact will certainly prove to be more rewarding than the same book without the dust cover. Check the spine's condition as well. A firm spine and pages intact are what a collector seeks.
Cookbooks are highly desirable to collectors.
The older cookbooks compared to modern day ones show how much society has changed in culture and eating habits. Collecting a particularly brand of cookbooks, such as The Joy Of Cooking, is a favorite among avid collectors. Another highly desirable series is any modern-day design book, which often go out of print quickly. These type of books are treasured for both their aesthetical and historical value. Children's books such as Nancy Drew first editions are a profitable find.