xpath: extract data from a node

Question

I am trying to extra some data from a webpage. the structure of the webpage is as below

<li id="yui_3_4_1_1_1326860702769_9706">
<span id="yui_3_4_1_1_1326860702769_9705">Sales rank: </span>
2
</li>

http://www.barnesandnoble.com/w/enders-game-orson-scott-card/1100353963?ean=9781429963930

I need to extract value "2" from above and identifier has to be "Sales rank"

Thanks for all the help.