Direct answer: Identify the exact edition and printing (using the number line and publisher statement), then search that specific edition's sold listings on AbeBooks or eBay's completed-listings filter — actual sold prices, not current asking prices, give the most realistic value estimate.
Why edition and printing matter more than age
Age alone tells you very little — a common 1960s book club edition can be worth a few dollars, while a true first edition of the same title in fine condition can be worth substantially more. Identifying the exact printing is the essential first step.
Condition drives value more than most people expect
Booksellers use a fairly standardized condition scale (from "poor" to "fine" or "as new"), and the difference between a reading copy and a fine, dust-jacketed copy of the same edition can represent a large multiple in price, especially for collectible titles.
Checking realistic, sold-price value
Current asking prices on marketplaces can be highly inflated by optimistic individual sellers. Filtering eBay listings to "sold" (rather than active) items, or checking AbeBooks' sold-price history where available, shows what buyers actually paid recently for comparable copies.
When to get a professional appraisal
For books that appear genuinely rare or valuable after your own research (early printings of major literary works, signed copies, or unusual bindings), a professional rare-book appraiser or specialist auction house is worth consulting before selling.
Does a signature always increase a book's value?
Generally yes, if authenticated, though the increase varies enormously by author and the significance of the specific signed copy.
Is a book club edition worth less than a retail first edition?
Yes, typically significantly less, since book club editions were produced in large runs specifically for a different market.
Where can I sell a valuable old book?
Specialist rare-book dealers, dedicated auction houses, and marketplaces like AbeBooks are generally better outlets for valuable finds than general resale sites.