I am trying to learn Bootstrap following this Orielly Bootstrap book.
This is the simple html code:
<title>Bootstrap 101 Template</title>
<link href="css/bootstrap.css" rel="stylesheet">
<div class="span8">Span 8</div>
<div class="span4">Span 4</div>
It looks like the book you are reading is out of date. That is the syntax used in Bootstrap 2 (which is no longer supported).
Presumably you have downloaded the CSS for Bootstrap 3 or 4. Consult their documentation for the current syntax, which has changed significantly since Bootstrap 2.