I think books have had by far the biggest impact for me to help improve my game. There’s a lot of content on the internet now that is very good, but most of it is so very much less comprehensive than a good book, which will often try to cover all aspects of play. You Tube pieces and even blogs and what not, tend to be more like an isolated page or chapter in a book. They can be very good, but I think it is easier to continue to have large holes in your game when you are always picking individual areas of interest out of the internet, rather than reading a good book on poker from cover to cover.
I’d also mention that I’ve found some books much better than others, and that in general I’ve found many of the books from the most famous professionals to be the ones I’d be least likely to recommend: Brunson, Negreanu, and Helmuth are all great players, and personally, I didn’t feel any of their books did much to help improve my understanding of poker. But everyone is going to feel different, and probably become a bit jaded by the first book or books that really do create some of the light bulb moments.