Perhaps the University simply feels that a person qualified to teach is capable of writing material with which to teach? Or that material written now would be more up to date in a fast changing field.
You must create the material to fit the curriculum goals for your class. Take the goals listed for your class in your curriculum. Use the goals as an outline, sort it in order of logical progression, then make each major goal a chapter. You can actually begin with each chapter being only a simple outline and expand it to paragraphs eventually as you teach the same material semester after semester. Remember to include an abundance of graphics. I created material that I have used the last several years at Adult Life Training and released it under a Creative Commons attribution license around 2003. It is still available for free download at alt-fw.org. That material has worked well for mature adults (>55 years), but would be far too simple for young college students with normal learning skills.