It would be very nice to have a ISBN plugin for Brazilian books. If you are willing to write the plugin (and I confess I would appreciate it), you could try to extract the ISBN data from Livraria Cultura's site (http://www.livrariacultura.com.br/scripts/busca/avancada.asp
). Livraria Cultura is the biggest Brazilian bookseller and, as such, has an amazing Portuguese book catalogue (including books published in Portugal). If this site doesn't work, you can try the ISBN search at the Brazilian National Library's site (http://www.isbn.bn.br/pesquisa-no-cadastro-isbn
). This one includes only books published in Brazil. I hope you agree to write the plugin.
(Many) thanks in advance,