See the syllabus for information about the textbook. The underlying mathematics in this course is quite old and there are many, many alternative expositions. A few:

- Chris McMurdie has extensive and reasonably well-edited lecture notes which may be used as a textbook replacement. We may cover topics in a different order, with a different emphasis, etc.
- Khan Academy has extensive material at this level, like this.

- Andy Loveless' most recent 307 course site includes a large amount of supplementary material. For instance, see the Exam 1 review, the Exam 2 review, and these slope field examples.