Hey man, i really like the piece. but like it was pointed out before, i feel it is far too busy.
if you squint and look at it, everything tries to get the same amount of attention. all the books in the far back are piercing the eyes, but dont really add to the picture all that much.
the variation in the closer bookkshelves is nice, it helps break up the monotony, and makes the library look used, good job on that.
but some of the details on them are too bright as well. its hard to determine whats foreground and whats background.
the thing that puzzled me is the light source. is it from above? from behind? the shadows and highlights are not clear in that. little highlights on the cheekbones, rimlight on the coat, light hits the book, casting a shadow of the head presumably, but the table is entirely in shadow. try to think where the lightsource is and how it hits everything.
that said, i feel like the mood of this piece is a bit too sober. If this is a demon warlock studying the mystics from the nether realm it feels a bit odd if he sits in fully eluminated room, but thats just my own take on it.
I tried to take this into account in the edit. Things i'd generally like to see you address are
focus: what is the point of the picture? the library, or the warlock?
posing and expression: your warlock right now looks pretty much like a hunched over student learning for his economy exam
design: aside from the horns there's not a lot that would actually identify him as a demon, satyr, and he might just be another dude.
Also i got rid of the far shelves in order to add some windows. it makes the image feel more open and less busy.
The candle light setting is a bit harsh and a big deviation from yours, admittedly, but i really wanted to try it out
Hope you can take something away from it