//This code will change ISBN links to point to the url of your choice,
//instead of Special:Book_sources. 
//
//How to use it:
//
//First, you must copy this code to your user javascript page. This is 
//at User:<your username>/monobook.js . My username is Lunchboxhero so
//my javascript page is User:Lunchboxhero/monobook.js . You must be logged
//in to edit your javascript page.
//
//Once you have copied the code and saved the page, you need to refresh your
//browser's cache. For Mozilla/Safari/Konqueror: hold down Shift while clicking 
//Reload (or press Ctrl-Shift-R), IE: press Ctrl-F5, Opera: press F5.
//
//The code should now work, as long as you are logged in.
//
//You can change the destination url by simply modifying the quoted url that
//follows document.links[i].href=
//
//Good luck, and if you have any questions, leave me a comment on the discussion
//page of User:Lunchboxhero/monobook.js .
//
//(Every line that starts with "//" is a comment and can be discarded.)

function externISBN() {
  for (var i = 0; i < document.links.length; i++) 
    {       
         var ln = document.links[i].href.match(/isbn=(.*)/);
         if (ln) {
           document.links[i].href='http://www.amazon.com/exec/obidos/ASIN/'+RegExp.$1;
         }
    }

}

$(externISBN);