If you are talking about "How To..." books versus "DIY..." videos, then there is probably some grounds to argue that YouTube is biting into book sales. But for any process which takes more than 5 minutes to master, or for anything that needs to happen more a few feet from your computer's screen, I think that even that genre still has a purpose.
As for the rest of the realms of non-fiction, nothing beats a book. The longest television documentary on any subject pales before the depth and completeness of a competently written textbook. Using a video as a reference document requires scrolling to the right spot in the timeline and the home pages of few YouTube videos come with time-stamp indices.
As for fiction, radio, cinema, and television have been attacking the dominance of the written word for decades. YouTube brings nothing new to that battle.
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