The problem with YouTube videos is they don't go into near enough depth; the majority just skim the surface of a topic or sub-topic and don't provide the concepts and foundations you really need. Books are good, but you don't get all the logic and caveats you really need from books alone; I have a whole stack of books and videos on lighting - it's good to have a small library of them on hand.
Many experienced folks will reference
Light, Science, and Magic - which I think is an essential reference on light to have in your library. If you are into speedlights, Joe McNally's
Hot Shoe Diaries is another good book, but I prefer a pair of books on speedlights written by Neil van Niekerk -
On-Camera Flash and
Off-Camera Flash, each of which provide a basic understanding and foundation. By the way, Neil is the inventor ot the
Black Foamy Thing (BFT) - the cheapest and one of the most useful light modifier for a speedlight.
If you want a lot of good information for free, go to the Stobist blog (
https://strobist.blogspot.com/). For a place to ask questions and get a whole bunch of responses (some good, some not, but you'll get answers, go to the l
ighting discussion forum in dPreview.
If you are into studio portraiture the best book I own isn't even a book - its freaking thick lighting
manual called
Studio Lighting Made Simple by Scott Smith that you can't buy online anymore from Scott, but I see one copy is available used on Amazon (and three times the original price used - that's how good it is:
https://www.amazon.com/Studio-Lighting-Simple-Scott-Smith/dp/B01DSGLCVI It's worth the price, I'll never part with my only copy, and want to be buried with it.
But I've saved the best for last. The best $49 you could EVER spend on an real studio lighting course is on this online class taught by Tony Corbell,
Light Shaping Tools for Professional Photographers.and any of his other classes on Creative Live (I've seen the class go on sale for $29 on occasion and have since taken an annual subscription for virtually all of the classes on Creative Live- a gold mine. I took a variety week-long in-person training classes over the year from several top pros including Tony. ony is someone in the industry who may pros consider one of the best lighting teachers in the world. Tony learned from the lighting greats - Dean Collins, Monte Zucker, and most of the the top pro portrait photographers that we'd often see take classes/sessions with at the annual State and regional professional photographer conferences. You won't be disappointed - no YouTube video can match the many modules and many ours you get (including lifetime access). Tony is just the clearest, easiest to understand lighting instructor you'll likely ever experience, hands down. .
Regards,
Mike