practice, practice,practice. Allways remember your basics. Is everything there that constitutes a normal sinus beat? A P -wave , qrs, t. Does it map out from r to r . does it have a t wave. If more than one p, how many and its configuration. qrs is it normal wide or bazzare, does the r-r map out or is it coming at different times. Is the t present. These are all things to look at no matter what lead they are in . Know that each rhthym has a change in one of those things and the heart rate. Know them well and you can figure each rhythm. I didn't need a book , but they say that dubin is good.