Initially it was library books and the now long defunct Photo News Weekly that ran a darkroom skills series. I had a darkroom (bedroom) before I had a camera. Big influences were An eye for a bird by Eric Hosking; Flying Birds by David and Katie Urry and Tir a' Mhurain by Paul Strand.
Ansel Adams was added later to this short list of influencers as his writings transformed the way I thought, about, and practised, photography by forcing me to think beyond the technical requirements of a photograph. Adams is replete with quotable sayings, but as a flavour here is one (that was easy to find) from the 1st edition of his Camera and Lens book.
"Photography is more than a medium for factual communication of ideas; it is an exalted profession and a creative art. Therefore. emphasis on technique is justified only insofar as it will simplify and and clarify the statement of the photographer's concept.."
Of course Adams put a lot of emphasis on technique, but it was always in terms of achieving the technical skills needed to allow the photographer to produce photographs that matched their creative vision.
Now a days, with digital cameras/smart phones, I think you can delay the technical mastery that Adams promoted and focus initially on making pictures and learning about photographs and photography. Only moving onto developing technical expertise when the lack of technical know how prevents you from "simplifying and clarifying" the statement you want to make with your photograph.
Eventually, my schoolboy hobby developed and I started to work weekends/school holidays, later full time, at my local professional and learned on the job (small team of photographers which did pretty well every type of photography). Switched job to one in an MOD research establishment and again worked with a small, but larger, team of photographers, (industrial and scientific photography mainly) and carried on learning on the job until a change in career.
I also did a 5 years day release studying photography, and multiple short courses, from one day Kodak marketing or Sinar how to use camera movement courses, to a two week video production course.
When, as a now amateur photographer, I got into digital and needed to learn a whole new collection of "technical" things I relied on books, and "paid for" online courses. Youtube is great, but when learning, as opposed to searching for answers to specific problems, I prefer properly structured courses.
Ongoing learning about understanding and appreciating "photography" as opposed to learning photographic techniques, is still mainly from books, a few selected youtube channels and a few online courses. edit: I should add that I subscribe to "Black and White photography" and "Studies in Photography" magazines.
There was a recent thread on here about books to help understand and appreciate photography:
I'm not sure how to define this, I'm interested in recommendations for books that are more about the "why and how" of photography in the broadest sense, than the "how to" of taking photographs. EDIT: Studying books of photographs (e.g. monographs) are a crucial part of learning about...
www.talkphotography.co.uk