I agree that his Special Relativity work was probably worked on with some contribution of Marić, but I think his later work was more individual. Others may deserve credit for some of it though http://en.wikipedia.org/wiki/Relativity_priority_dispute

History has a way of attributing contributions to the foremost scientist of the era. For example, "Newtonian" physics implies and is often taught to have all been first discribed by Newton, when in reality it was a joint effort of him, Galileo, Kepler, Descarte, Hooke, Leibniz, and others.