function book(id){
 if(id=='del')
  document.getElementById('del').src = 'img/book_delicious_1.png';
 if(id=='folkd')
  document.getElementById('folkd').src = 'img/book_folkd_1.png';
 if(id=='linkarena')
  document.getElementById('linkarena').src = 'img/book_linkarena_1.png';
 if(id=='google')
  document.getElementById('google').src = 'img/book_google_1.png';
 if(id=='wong')
  document.getElementById('wong').src = 'img/book_wong_1.png';
  }
  
function nobook(id){
 if(id=='del')
  document.getElementById('del').src = 'img/book_delicious.png';
 if(id=='folkd')
  document.getElementById('folkd').src = 'img/book_folkd.png';
 if(id=='linkarena')
  document.getElementById('linkarena').src = 'img/book_linkarena.png';
 if(id=='google')
  document.getElementById('google').src = 'img/book_google.png';
 if(id=='wong')
  document.getElementById('wong').src = 'img/book_wong.png';
  }