ISBN 839105425X is nu­me­rically correct (check­sum OK).
The ISBN 839105425X that you input was found to be numerically correct. This only means that it is 10 or 13 digits long and that the last digit (the checksum digit) has got the correct value according to the checksum formula. Thus the ISBN cannot have been produced from a valid ISBN by a single error (change in one digit or transposition of two digits). This check does not prove that the ISBN has ever actually been assigned to a book. As a way to check it the ISBN corresponds to a book this site offers links to the following bookseller's sites: amazon.com (USA), amazon.co.uk (UK) und amazon.de (Germany). As the prefix of the ISBN is 83 the corresponding publisher is part of the following prefix group: Poland	To the home page
	Diese Seite auf deutsch Tiu ĉi paĝo en Esperanto (Unikodo) Tiu cxi pagxo en Esperanto (x-igata)

If these book sites do not know the ISBN this does not mean that there is no book with that ISBN - the book could have been issued decades ago, be from another country than those catered for by the Amazon sites, or it could be of minor commercial interest to those large bookshop sites. Additional sites to search by ISBN (in this case, in catalogs of libraries) are e.g. the Library of Congress (USA) or the search gateway of the Deutsche Bibliothek.

You can check another ISBN with this form.