Can I use my textbook as a reference?

My bottom line answer to this is: ask your professor.  My advice below might not be correct.

If it’s too late to ask your professor or you don’t feel comfortable doing that, the usual answer to this question is no.  Your textbook is there as a reference tool, as a guide to the subject area, and as an introduction to your course.  But usually (depending on the course) textbooks are not appropriate resources for your course assignments.

If your textbook is super helpful, though, try tracking down the resources listed in its bibliography.  Don’t just cite these resources, though, make sure to actually find them!