I'm not sure if the overall prevalence on the precentage of people who have "arrhythmias" is known or has even been studied, since many "arrhthymias" have little to no clinical significance and people go in and out of arrhythmias all of the time. Anyways, of course the incidence increases as people age, and for one of the most common clinically significant arrhythmia's is A fib, whose prevalence rises to ~10% in the population who are over 80. Premature ventricular contractions, technically an arrhytmia, has a prevalence of anywhere from 20-50% in adults (higher in those with CAD), and 2-4% in children. Anyways, a few good cardioelectrophysiology books are Dubin: Rapid interpretation of the EKG; and the cardiologists I have been working with reccomend Marriottt's Practical Electrocardiography. Neither discusses prevalence of arrhthymias though, they only discuss how to diagnose them using ECG's.