This is what the experts, specifically quoted from www.skinnerinc.com, Selling Antique Books, Part II, had to say:
" 1. Dust jackets, dust jackets and more dust jackets!
If there is one single thing that is a make or break for book value, it would be the dust jacket. The value of a first edition copy of F. Scott Fitzgerald’sTender is the Night with dust jacket is around $6,000. Without a dust jacket….$300. This huge difference in value is largely due to the fact that more than 90% of dust jackets are destroyed, either deliberately or due to their ephemeral and fragile nature. If you have one on a good book, treasure it. Also, be sure to protect it with a plastic sleeve."
Unfortunately, I learned this the hard way. I purchased Carl Sandburg, Poems of the Midwest from an estate sale. I was very excited to get it because there were no markings, no creases, no tears, no stains and NO DUST JACKET! Even though this is a very desirable 1946 First Printing Edition, its value went from around $150-$200 to well, I'm selling mine for $20 bucks. :(