Figure out the condition of your book. AbeBooks.com is one of the premiere online sites for buying and selling used, rare and hard-to-find books. AbeBooks lists seven possible conditions for a book:
New
As New
Fine
Very Good
Good
Fair
Poor
Depending on where you want to sell your book, you'll need to check with a particular seller's glossary of book conditions.
What edition is your book? First editions tend to be pricier than later editions. Hardcovers tend to be pricier than paperbacks.
This is where a good collector's guide comes in handy. It can tell you what the standard market price is for each edition. Or go online to a site such as eBay.com, Amazon.com, or AbeBooks.com and search for your book's title and edition to learn what other comparable copies are selling for.
Is your book signed by the author? Signatures that are hard to come by are worth more than standard autographs--for example, if your author is a known recluse. Some authors do book tours and signings, and their signatures therefore aren't worth as much as authors who rarely sign books.
Taking all of this into account, price your book. Start with the lowest possible price you think your book would sell for based on its condition, edition, and author signature. Then figure out the highest price you think it would sell for. Your list price, then, is the exact middle of the lowest and highest price --the book's fair market value.