Ansel Adams‘ „The Camera“ and „The Negative“ contain some very valuable information.
Photography
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Search for a "rule of thirds tutorial".
I am well aware of the rule of thirds. I was hoping to deepen my knowledge and skills a bit beyond putting the subject on the third-lines. Thank you for your suggestion though :)
Or maybe the rule of thirds is deeper than I thought? I am no expert..
I am no expert either. I have just looked through ~10 of your posts and got the impression: that's one of the most obvious missing things.
Two short non-technical books that are well worth a read:
On Being a Photographer: A Practical Guide - Bill Jay and David Hurn. Written well before the digital age but still very relevant
Art and Fear - David Bayles andTed Orland. Aimed at anyone involved in creative fields, it explores your relationship with what you do