How to sell or value an old book

I am not a bookdealer, but the following information may help you find what you're looking for.

The value of second-hand books is strongly dependent on their condition, which the trade (at least in the UK) describe as:

Mint - as new
Fine - almost as new
VG (Very Good) - showing only minor signs of use
G (Good) - bookseller-speak for well used
Poor - text complete, otherwise defective: a reading copy only

and whether it has its dust-jacket/wrapper & the condition of that, and also whether it's autographed by the author & if the dedication is to someone important.

Next, you need to identify the edition you have. To help do this, look it up in the US Library of Congress catalog, for British authors also try the British Library, for Canadian authors also try the National Library of Canada, for Australian authors also try the National Library of Australia, for New Zealand authors also try the National Library of New Zealand and for French & other European authors also try the Bibliotheque Nationale de France.

Thus armed with the publishing data (full title, full author(s)/editor(s) name(s), publisher, place & date of publication) you can check what people are asking for other copies through the price-comparison services at AddALL BookFinder.com, bookfinder4u, BooksPrice.com Used Book Search .co.uk or Used Book Search .net, or the comprehensive book mart systems at abebooks.com & Alibris. Or you can offer it for sale or ask for a valuation from a second-hand or antiquarian book dealer specialising in a relevant subject from the extensive list in the AcqWeb Directory. Alternatively, try a local second-hand or antiquarian book dealer, who you may be able to find in the Yellow Pages of big cities.