In Mail, Preferences, Viewing, there is a menu called "List Preview" which shows how many lines of a message to show without actually reading the message. Also, you can just close the preview part of the window (double-click the left hand border of it). Then you can click on messages - selecting them - and not read them. You have to double-click on them, or press return to read them. And they open in their own window.
Also, if you read one unintentionally, just select it and press command-shift-u to mark it unread again.