Extenda

This is assuming you are new to Delta and not just switching planes: if you want to start studying for initial qualification, the best training resource are the ebrief videos by far. The ESV is your intial systems test which you pretty much just do until you pass and you can study that by doing the PowerPoints they assign to you.

I’d be cautious about using third party study guides. I’ve never really heard of anyone using them. Delta gives you way too many training resources as it is they’re hard to keep track of.

Delta training is not hard.