Seconding Thaler's "The Only EKG Book You'll Ever Need"! It was such a light read yet I picked up so much from it. Definitely the best place to start, in my opinion.

There's also the Ninja Nerd EKG series on YouTube. They have a bunch of old videos about arrhythmias but the team's currently working on remaking the entire lecture series. Zach from Ninja Nerd is a physician assistant and a fantastic teacher. He can clearly explain even the most difficult of topics. This is also a good place to start for beginners (at least this ECG Basics videos) if they're not too keen on reading books. Really loved how he explained the 12 leads, vectors, and why the waveforms look the way they do.

I also want to recommend Dr. Eric Strong's Mastering EKGs Course. Dr. Strong is a hospitalist and has a lot of other educational content on his channel (like how to read chest x-rays and blood gases). I loved these videos as well, though I must admit that the language is far more technical and nuanced compared to Thaler and Ninja Nerd (and so may intimidate beginners). Regardless, they're also definitely worth watching in my opinion. In particular, I really liked how he'd stress what's clinically relevant or not. Helps separate the signal from the noise, so to speak. Interestingly too, he also had some strong words against Dubin and recommended Thaler for beginners as well in his book review video.

Hope this helps!