function amazoncom(isbn)
{
  location.href="http://www.amazon.com/gp/reader/"+isbn+"/";
}

function google(isbn)
{
  location.href="http://books.google.de/books?vid=ISBN"+isbn;
}

function amazonde(isbn)
{
  location.href="http://www.amazon.de/gp/reader/"+isbn+"/";
}

function libreka(isbn)
{
  location.href="http://www.libreka.de/"+isbn+"/FC";
}