The most common reason to have a book or book collection appraised is to get an official determination of the fair market replacement value of the book(s) for estate, insurance or charitable donation purposes. An appraised value will typically be the average market price for a comparable copy of that book on the market currently. The desirability and scarcity of an item are key factors, as well.
Our staff has been appraising collectable books for more than a decade, with over 50 years of combined experience. We have evaluated books worth a few dollars to collections valued at hundreds of thousands of dollars. Our fee for the appraisal services we provide is $125/hr.
We are happy to provide informal estimates for a few books free of charge, although we encourage you to try your hand at research first. Two sites we frequently use to help us evaluate books are abebooks.com and vialibri.net. To learn more about what goes into determining a book's value, we recommend this article from Biblio.com.