The answer you need is not in any program or book, the answer lies in and with you. In dealing with colleaques who had attitude, i learnt that i had to firstly deal with self: ensure that I was not over reacting to them. we all are from different environment, backgrounds, had diverse upbringing, exposure, etc the list is endless. In pursuance of survival or gainful employment, we were holed up in this pretty work house to perform miracles - deal with customers who see no reason for your service(s) or not so good product (until you are able to prove otherwise), who may not be in the mood to supply adequate information for you to offer the necessary assistance. Do not get me wrong, a number of our customers are lovely people and are willing to give the required support.
the picture i am trying to paint here is that you are already in a lose position before the client comes in (complaint) and in a defensive position (enquiry), in both scenario, you are under some pressure - to satisfy the ego of this person in front of you.
Learn to be firm, soft, playful, sweet, joke, smile, laugh, snobbish, ready to constantly change personality by the second. Exhibit confidence, let them know they can trust you, show them the correct thing to do when they are wrong and do not be tired of repeating yourself. empower them with knowledge, let them understand the expectations of management, communicate (gradually disseminate useful information to them being careful not to release sensitive details and ensure it is the truth), your actions and decisions must be human. You can never be given all you require, whilst trying out some of these tips other good ideas will surface.