There are a multitude of books and websites out there on ECGs – you only need to Google it to find anything you’re looking for. Sometimes though, you need a simple, clear explanation of certain parts – like interpreting the axis.
The ECG library looks pretty ancient, but has a few good examples of some classic ECGs. What we like especially about this site though is the explanation of axis, with a simple trick to be able to interpret it quickly. Have a look at the site here.
After you’ve got the basics down and just want to test yourself out, then Wave-Maven is the place to go, a site run by Harvard. Over 400 ECGs with a clinical vignette, with varying levels of difficulty. Check it out here.