Among possible sources of readings, I would mention the booklet How to Teach Mathematics by Steven G. Krantz. It is quite practical, and it costs very little.
By the way, one of the other responders mentioned an unpleasant issue. I would like to mention another issue that is potentially much more unpleasant: The TA should keep the door to the office open when a student comes to office hour, most particularly if the student is of the opposite sex. Very often the student closes the door when entering the office, even if the door is open. In such cases, I tell the student to keep the door open "in case someone else comes along." The unpleasant developments that this precaution prevents are extremely rare, but when they occur, they cause quite exceptional amounts of trouble.
