1. Smithsonian Libraries (https://library.si.edu/departments/special-collections/information-old-books):
This page contains very easy to understand information about the care and pricing of old books. At the top are several links to subsections of the page, two of which are particularly helpful. “Appraisals” will take you to a section that includes links to professional appraisers in the United States. If you don’t want to contact a professional, the “Do It Yourself” sections will help you deal with the most common situations. One important takeaway is this:
“About market values, a word of caution: the monetary value of a book is what someone else is willing to pay for it. The demand for any particular author or title may fluctuate up and down over time. In addition, any individual copy's value will vary on the basis of its edition, scarcity, condition, completeness, and association. Any valuation or appraisal is an educated guess, applicable only to a specific copy at a specific moment in time.”
2. AbeBooks’ Guide to Conditions (https://www.abebooks.com/books/rarebooks/collecting-guide/understanding-rare-books/guide-book-conditions.shtml)
A large online marketplace for books (and a subsidiary of Amazon since 2008), AbeBooks provides a very useful guide for determining the condition of a book. Includes not only basic definitions of the various grades of condition, but also some of the terminology you will often find in descriptions.
1. The Antiquarian Booksellers Association of America (ABAA): https://www.abaa.org/
2. The American Society of Appraisers: www.appraisers.org
3. The Appraisers Association of America: www.appraisersassoc.org
4. The International Society of Appraisers: www.isa-appraisers.org
Each of these professional associations offers users the ability to search for appropriate experts in their area.