The bookstore is selling Zill First Course in Differential Equations with Modeling Applications, 10th edition. You are not required to buy this book. Any edition of Zill will do fine. In fact, any textbook on differential equations written in the last thirty years will cover the same topics (perhaps with different notation, but you should be capable of adjusting for that).
If you use another text, you should be able to find appropriate readings by matching the lecture topic to your text's table of contents or index, with the following hints