I used Garcia's 12 lead EKG book and really liked it. Starts of with basics and has 3 different levels of complexity so you can start on basic and hen go back and add on some mid and expert level interpretation.
The best aspect i thought is that it was a ton of 12 samples, so it will introduce a concept, such as ST depression/elevation and then walk you through a half dozen examples.
There are better books if you want a lot of detailed physiology and theory behind ECG but I've found for reading ECCS in a clinical context nothing is better than practice looking at actual 12 leads.