There's lots of basic ECG interpretation stuff out there... just google... Uhhh... "Basic ECG interpretation". If you've gotta have a book, Dubin's "Basic Interpretation of EKGs, 6th Ed" is what you want.
Interestingly enough, AEMTs in PA, who can run a code with an iGel, IO and Epi are specifically NOT allowed to interpret rhythms and must use the monitor in Advisory mode. (This is ridiculous to me, but if you know anything about PA, it's not surprising)
Also, EMTs may use the cardiac monitor for SpO2 and NIBP but may NOT place leads on a patient for continuous ECG monitoring. That's a quick way to getting your cert pulled if you tick off the wrong doc. See: Cocky EMT brings in a patient from a nursing home on a monitor, (without a medic) and tell the doc he's wrong about the rhythm. (Just recently happened here)
If you have a medic with you, the monitor should NEVER be in advisory mode.
If it's a basic, it should ALWAYS be in advisory mode.
Of course, YMMV, and you should talk to your operations and/or QI people to determine what's appropriate behavior in your agency.