When you have a message highlighted, you can do plenty of things with it (besides reading it, of course). You can print it, save it to a file, move it to another folder, delete it, and more. Most of these operations are straightforward, so I'll just summarize the basic techniques here:
Reading other messages: If you have several messages you want to read, you don't have to return to the message list to navigate your messages because the View | Next submenu gives you the following commands:
Next Message: Moves forward to the next message (the one below the current message in the message list). You can also press Ctrl+>.
Previous Message: Moves back to the preceding message (the one above the current message in the message ...