ems12lead.com is pretty good too. TomB, a member on here too, has a bunch of case reports and guides on EKGs/12-leads.
Yeh, I definitely recommend Rapid Interpretation of EKGs by Dale Dubin too. I've also heard good things about 12-Lead ECGs for Acute and Critical Care Providers by Bob Page and I think it was 12-lead ECG: The Art of Interpretation by Thomas B. Garcia too (I can't see the picture online right now, but I think that's the one).
I've read several books, mastered the block method, can draw and label the heart in less than a minute, learned some formulas, etc, but a friend (Tommerag) showed me three strips, and it took me two tries to get them right, and he gave me a hint for one them. I thought I mastered rhythms, but it turned out I didn't. He had a packet/book of different ones from the field. Something like ECGs for Emergency Physicians 1 and 2 will help you practice interpreting rhythms to make them less scary and help distinguish between artifacts and the different waves.
Y'know, EKGs and 12-leads are fun, but it's just a skill. If you plan on becoming a physician and you haven't read: biology, chemistry, ochem, immuno, micro, biochem, anatomy, physiology, etc., those might be more useful to you both as an EMT right now, and later on as a physician. The only useful thing I can think of with an EMT knowing how to interpret EKGs is passing drugs to their paramedic partner. :\ I love EKGs and 12-leads, but if I could press the redo button, I would've read the other stuff first.