为什么我的代码不可以自动居中？
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<style type="text/css">
*{margin:0;padding:0;}
#wrap{width:770px;margin:10 auto;}
#header{width:100%; height:200px;border:1px solid red;}
#mainbody{width:100%; height:200px;border:1px solid bule;}
#footer{width:100%; height:200px;border:1px solid green;}
.content{border:4px solid #badbdb; padding:44px 15px 15px;width:770px}
.book{ background:url(file:///E|/网页布面布局/网页布局基础/case2/t_book.gif )no-repeat #eff9f9;}
img{margin:10px 18px; border:1px solid #b1adad;}
</style>
</head>
<body>
<div id ="wrap"></div>
<div id:"header">
<div id="mainbody">
<div class="content book ">
<img src="file:///E|/网页布面布局/网页布局基础/case2/images/book1.jpg" width="80" height="80" />
<img src="file:///E|/网页布面布局/网页布局基础/case2/images/book2.jpg" width="80" height="80" />
<img src="file:///E|/网页布面布局/网页布局基础/case2/images/book3.jpg" width="80" height="80" /><img src="file:///E|/网页布面布局/网页布局基础/case2/images/book4.jpg" width="80" height="80" /><img src="file:///E|/网页布面布局/网页布局基础/case2/images/book5.jpg" width="80" height="80" />
</div>
</div>
<div id ="footer"></div>
</div>
</body>
</html>
