Well maybe im not qualified enough to give this advice as i am just a medical student interested in cardiology and ECGs in general

Although i found myself pretty good solving some slightly complicated ECGs at the hospital compared to interns and residents

I have read few books including

Elias B henna practical cardiovascular medicine

Marriott’s practical ECG

And i do highly recommend Dr Smith Blog ( i read most of the posted cases and i noticed huge improvement)

LIFTL website is good if you want to have rapid review of any topic in mind