mail: how to read message without new window

Hey all, been a while for me, thanks in advance for the advice.

Problem: I until very recently read my e-mail in a box on Mail that appeared under the the lists of e-mail in my mailbox. I could then select another email and the box would show that e-mail. Now the box is gone (not sure what happened) and all I see if a long list of emails in my mailbox. I can read them when I double click on them (it opens up a new window), but I would like to go back to the way it used to be. I hope that made sense. I have tried a few of the settings under File, Edit and View in Mail but can't seem to figure it out.
I wanted to take a screen shot but don't remember how to do that either.