I'm not it's the menu key on the reader? It's virtually impossible to navigate around the SONY reader without using it? Are you referring to the button that is located under the "Menu" text? Or the big button in the middle of the arrow wheel?
What that button does is navigates you up to the menu selection of the eBook. Also holding the button down for 1 sec will take you to the home page. A very handy trick.
BTW you should not be afraid to press any button. The SONY should be able to handle all button presses.