I personally prefer "12 Lead ECG: The Art of Interpretation" By Garcia and Holtz. It is filled with examples and exercise strips. The manual is written with 3 different levels in it. You can read the beginner level and then go back and do the other two levels, or you can start at the advanced level, it's up to you!
Here's the site:
http://12leadecg.com/
Also good on you for trying to advance your knowledge of ECG's. Don't let anyone on this board deter you from bettering yourself as a medic!