I like the TidalGardens Youtube channel for coral spotlights. I also really like the 101 marine invertebrates handbook that microcosm put out. After that, it's also a lot of googling.
As an example, let's say that I see X coral for sale. I think "wow, that's a great coral! Maybe I should buy one!" First, I'll check the 101 marine inverts book I have, then all the other books/magazines I have to see what information there is that's printed. Then I'll look on Youtube and type in the coral's name. If TG has put out a video, I'll watch it on 2x speed. That usually gets me up to beginner level of whatever it is that I want to keep, and then I can make a semi-informed decision about if I should buy it or not.