A few selected resources on heart rate variability (HRV) with comments:

HRV 101: A few of my own slide decks as an intro to HRV (including some info on acquiring heart or pulse rate during MRI) – English or German.

Classical “consensus” paper 1: The 1996 “Task Force” paper Heart rate variability – standards of measurement, physiological interpretation, and clinical use. European Heart Journal 17, 354-381

Classical “consensus” paper 2: The 1997 Berntson et al. paper Heart rate variability – origins, methods, and interpretive caveats. Psychophysiology34(6), 623-648.

A set of very useful newer tutorials and resources by Dan S. Quintana and colleagues: Considerations in the assessment of heart rate variability in biobehavioral research (2014, Frontiers in Psychology), Guidelines for reporting articles on [psychiatry and] HRV (2016, Translational Psychiatry), and Statistical considerations for reporting and planning heart rate variability case-control studies (2017, Psychophysiology).