Professional standards, University policy, and IRS regulations prohibit the Special Collections staff from making monetary appraisals.
A list of antiquarian book dealers who will appraise your book for a fee is usually available in your local yellow pages under Books--Used & Rare. If you live in Colorado or Wyoming, the Rocky Mountain Antiquarian Booksellers Association website (http://www.rmaba.org/) may also be of help in locating an appraiser.
To obtain a general idea of how much a book may be worth, it can be helpful to look for similar books for sale on the web at places like
http://used.addall.com/, http://www.bibliopoly.com/, or http://www.abebooks.com/. These databases provide access to the stock of national and international book dealers. Condition is very important when comparing your items to the listings.
The Rare Book and Manuscript Society publication "Your Old Books" has answers to many common questions about determining rarity of a book, care for old books, and appraisal.
Special Collections and the Art and Architecture Libraries have recent Sotheby's catalogues.
Special Collections has the index American Book Prices in both print and CD-ROM, California Book Auction Galleries (which ceased publication in 1999), Annual Register of Book Values (ceased in 1999), and Bookman's Price Index.