The coverage of epigenetics in Intro Psych textbooks appears to be slowly on the rise. And with good reason. If you're not familiar with epigenetics, this 9-minute student-friendly video is a nice introduction
For a more scholarly introduction to epigenetics, this 2016 article from Child Development will get you up to speed.
In Intro Psych, your textbook may give an overview of the topic wherever it covers genetics and revisit epigenetics again during coverage of psychology disorders. Research is stacking up. Our experiences influence the turning on and off of genes that are linked to psychological disorders. For example, "Exposure to stressful or traumatic life events, especially early in life (early life stress (ELS)), is one of the strongest risk factors for a number of psychiatric disorders, ranging from post-traumatic stress disorder (PTSD) over depression to bipolar disorder and schizophrenia. Over the past decade, an ever growing body of evidence indicates that exposure to stressful life events can lead to long lasting changes in a number of systems including the endocrine system, the immune system and brain structure and function" (Provencal & Binder, 2015).
If a cause of psychological disorders is related to epigenetics, the effectiveness of treatments may also reside in epigenetics. Electroconvulsive therapy, for example, may alter epigenetic tags (Jong, et.al., 2014). Psychiatric drugs may also work this way (Boks, et.al., 2012).
For Intro Psych, the specifics of epigenetics is probably not that important, but a broad overview and the implications of the research are certainly worth the time.
Boks, M. P., de Jong, N. M., Kas, M. J. H., Vinkers, C. H., Fernandes, C., Kahn, R. S., … Ophoff, R. A. (2012). Current status and future prospects for epigenetic psychopharmacology. Epigenetics, 7(1), 20–28. http://doi.org/10.4161/epi.7.1.18688
Jong, J. O., Arts, B., Boks, M. P., Sienaert, P., Hove, D. L., Kenis, G., . . . Rutten, B. P. (2014). Epigenetic effects of electroconvulsive seizures. The Journal of ECT, 30(2), 152-159. doi:10.1097/yct.0000000000000141
Lester, B. M., Conradt, E. and Marsit, C. (2016), Introduction to the Special Section on Epigenetics. Child Development, 87: 29–37. doi: 10.1111/cdev.12489
Provencal, N., & Binder, E. B. (2015). The neurobiological effects of stress as contributors to psychiatric disorders: Focus on epigenetics. Current Opinion in Neurobiology, 30, 31-37. doi:10.1016/j.conb.2014.08.007