Books. The problem I see with YouTube is that nothing is vetted. Books have editors to double check things.
I'll agree that books are more likely to be researched and vetted, but there's certainly no guarantee of accuracy. Especially for something like brewing, which is still a relatively small niche market where a small publisher may not have the expertise on the topic. My wife's coworker self-published a book on the Civil War. I don't think an editor was even involved with that.
Yes, you've got the big names who you can reliably count on, but those books typically focus on a specific topic or they tend to be like textbooks for brewing, with a lot of general info and guidelines. Perhaps you'll see a picture or description of the brewer's own setup, but there are so many different manufacturers out there and even more permutations for putting a system together. I can see a variety of different equipment and builds posted on forums or on YouTube, which can give me ideas for optimizing my own setup.
Also, books are still static, printed text. You can't see an actual demonstration of a technique or a brew system in action. Sometimes, seeing how something is actually used can really help a new concept to "click".
I appreciate the huge amount of bad info that's potentially out there, and you need a critical mind anytime you're on the net. But again, I feel that YouTube fills a need for me.