I only have a few options for my letters, and one of them is from an adjunct faculty member for our engineering department. He has been with the school for almost two decades and does not have his PhD. However, he did get his masters at Stanford. Does his letter lose credibility/impact since he isn't a full-time professor and does not have a PhD? I am close to the instructor and feel as if he will write me a strong letter, as I've had him for two of my classes that I've excelled in, and he was also our honor society's adviser. Should I look elsewhere for a letter?