Yes it is possible to pass any examinations with utmost sincerity and dedication without any kind of coaching. However, according to me, any form of coaching from a good institute will make your chances brighter. In your case, as I assume you might be spending most of the time in your job. It is not easy to maintain a regular schedule of preparation for a very competitive examination like IES. Also merely following a regular schedule on a selected books may not be sufficient. Remember,it is just like civil services examination where students, academics and people serving in private and government sectors compete with each other for a limited number of seats. Thus it requires not only regular studies, but also up to date knowledge about the latest trends. By joining a coaching a reputed coaching institute you will not only get a ready made course material but also get good guidance. Various mock tests organised by the couching institutes will also help you to prepare well ahead of the actual examination date.
Finally it is up to you how you want to prepare for this one of the toughest examination. You can initially start appearing in other related examinations organized by public sector organizations. Clearing these examinations will boost your confidence for IES.
Tulika Devi Nath