Pricing old books involves a great deal of research and observation. Even old and rare book experts typically find it necessary to contact other experts in the field to establish an estimated value. It's best to always work with objective neutral parties such as librarians; they can direct you to more information rather than providing you with a potentially inaccurate estimate.
Assess the book
Determine the condition. Compare it to other books, particularly those of the same age. Note specifics about the book such as missing jacket, faded title or ripped pages.
Consider location. A book on a local or newsworthy subject will command a higher price in the right place.
Investigate rarity. Search online and speak with your local librarian or other book collectors about how many copies of the book are available in the same printing and condition as yours.
Determine demand. A hard-to-find or old book isn't valuable unless there is someone out there willing to buy it.
Look for the print date and edition. Typically, you'll find this information on the first printed pages. However, some old books will not have this information and you'll need to do further research. If you can locate the publisher but not the print date, try contacting the publisher (or the company that has since acquired the original publisher).
Look for positive attributes. A true first (first edition/printing), signed copy, world-changing information, artwork by a famous artist, or rare binding type add value.
Talk to your local librarian, especially if you have a book of local interest. Librarians are usually well-connected in the community. They can help you locate identification resources and perhaps recommend a reputable appraiser.
Consult a guide to valuing old books. The National Library of Scotland recommends The ABC for Book Collectors by John Carter and Nicolas Barker
Check other resources. Online resources such as biblio.co.uk can help establish an estimate of value.
Speak with other collectors in person or on a forum. Obtain multiple opinions.
Hire a reputable appraiser if you are unable to determine the worth of your book on your own. This is especially pertinent if you are looking to determine book value for insurance purposes.
Missing pages typically negate any positive qualities of the book. It's often difficult to determine the specific value of an old book without the help of a specialist. Within the field, some enthusiasts and appraisers specialise in types of old books by author or location.
Beware of any self-proclaimed experts willing to appraise the book without seeing it in person.