My initial stamp is still valid, but I know some people who've had to do it. No, you do not need an employment letter. Remember, you already are a Permanent Resident. Just bring your passport. One person I know brought the courtesy copy of the I485 Approval Notice, but it was not needed.
FYI, when your I-485 is approved you can (should) go to your local BCIS office and have them stamp your passport to indicate that you are a Permanent Resident. This is particularly important as it is proof of your status until the plastic card arrives. It can take up to 1 year (looks like it's getting longer) for the card to be issued these days.
rgds,
sadiq