If your have just switched to the new faster Yahoo Mail Interface, then you would have certainly missed one feature of reading pane. reading pane is a window just beneath the mail list. If you click on a mail, the mail content is shown in the reading pane.

This feature is available in the new interface but is not selected by default when you switch over to it. You need to manually set it to make your reading pane visible.

Select Options->mail options from top menu. Select 'General' from left hand pane.
Now, select the radio button 'Each panel separately' from the Scroll
Save the changes.



Yahoo will now refresh your browser window to reflect the changes. Once done, you would be able to see the reading pane as follows: