if (document.images) {
	img1off = new Image();
	img1off.src = "images/illustrations/book1.jpg";  
	img2off = new Image();
	img2off.src = "images/illustrations/book2.jpg";  
	img3off = new Image();
	img3off.src = "images/illustrations/book3.jpg";  
	img4off = new Image();
	img4off.src = "images/illustrations/book4.jpg";  
	
	img1on = new Image();
	img1on.src = "images/illustrations/book1_over.jpg";  
	img2on = new Image();
	img2on.src = "images/illustrations/book2_over.jpg";  
	img3on = new Image();
	img3on.src = "images/illustrations/book3_over.jpg";  
	img4on = new Image();
	img4on.src = "images/illustrations/book4_over.jpg"; 
}