I will be leading a discussion class on differential equations for engineers this coming semester and I am wondering if anyone has a book that they could recommend. The book that will be used in the course is the book by Boyce and DiPrima. I am looking for a supplementary book from which to take problems to solve in class. Is there a book that has conceptual but not overly theoretical problems for differential equations? problems which are insightful but not too complicated? These problems are not the sort of problems which test skills in algebraic manipulation (I know it's ironic given that this is an engineering class) but problems which reinforce understanding of the concept?