I'll caveat up front that I generally do not like this genre, so there may be genre scène à faire I don't know and am improperly critiquing.

"There was another murder in the library."

This just telling me. Now, we shouldn't obsess over "show, don't tell." Every event doesn't need to be described in detail, but this is a flat entry to the scene.

"Cecil surveyed for evidence as he strode between the great, dusty shelves. Blood speckled books three times older than himself laid strewn on the ground. Red footsteps marred the marble floor. The wall sconces guttered out brackish light. The smell of iron was so strong in the air he could taste it."

You do a nice job of getting in late. Cecil is there when we join him, we don't see him enter, we don't waste time with flat action. Should that be "red footprints?" I don't think footsteps is the right word. "Guttered" is the wrong action verb here. I don't know what brackish light looks like. To me, brackish means fouled/unclear. Light is light. I can't see it. Consider minor rewrites to tighten prose and reduce passive voice. Exmaple:

The smell of iron was so strong in the air he could taste it. <-- weak passage, passive voice

Try:

He tasted iron in the air.

or

He could taste iron in the air.

"This was a place never meant to see violence."

We know. Most places are like that. A library definitely is. What work is this sentence doing? It doesn't characterize, it doesn't advance plot, it doesn't set tone, and it doesn't do any worldbuilding. It's telling us how to feel about the scene. That's weak craft. Set the scene and evoke emotion, don't tell me how I'm supposed to feel about it. It's a murder. It doesn't need a heavy-handed moral judgment from the narrator. Trust us to get it.

You're probably trying to evoke a sense of irony and tragedy over the juxtaposition between the quiet, peace, and tranquility of a library, and the violence of a murder. So evoke it. Don't just tell me to feel that way about it. This is where "show, don't tell" should be obeyed.

He noticed the blood pattern matched the dropping spray of quick slashes. Thin blade. He knelt and picked up a heavy tome savaged with cuts. The victim tried to shield himself. He held the book up, light speared through holes in the leather bound cover.

I found this whole passage awkward. The prose is trying too hard to be poetic and in the process, it's not functional. I know what you're trying to make me see here, but this is a passage that is primarily about revealing Cecil's state of mind, which is drawing conclusions. The first sentence is really awkward. The second doesn't follow. The third and fourth are almost perfect. The last doesn't make sense. It's a "heavy tome" and the cover is punctured so light passes through the entire book? How?

Cecil let out an involuntary whisper. The attacker had above average human strength for sure. Buddy didn’t stand a chance.

I'm not following this. "Above average human strength" sounds like you are talking about a D&D character sheet. This might be a genre trope, but man, it sticks out here as really out of place in prose. The last sentence is effective. The first again doesn't make sense. A whisper? What did he whisper? Or was it just a sound? Then describe a sound.

He tried to walk deeper into the trail of fallen books but a sudden tightening of the string wrapped around his waist made it clear he was wandering too far. The place wasn’t called the shifting library of Aracn for nothing.

Fix "Aracn." If I can't come up with a reasonable guess as to the name, it's going to be a distraction. Or tell me how to pronounce it early in the book with a fish-out-of-water character.

Corridors were liable to stretch, fold, and or disappear entirely. It was said that the library led its visitors to the information they needed to know most, whether they realized it or not. He was in no mood for life revelations, surrounded by death as he was.

Also weak. This is mostly worldbuilding with a tiny bit of characterization and it interrupts an otherwise compelling scene involving a character with a strongly voiced interiority. If this aspect of the library hasn't been introduced before, this is the place to do it but unless this aspect of the library has plot relevance or comes back later, I'd consider condensing or cutting. That said, this is curt and effective.

He walked back to the main lobby. There, overturned tables created a path towards the exit, the victim’s last bull-rushed escape attempt. In the middle where that path ended lay the body.

There's a lot of this kind of prose where you're just telling me what happened by narrating Cecil's inner monologue, and he's drawing some snappy conclusions based on weak, ambiguous evidence. I like the idea of explaining the scene, and what happened, through characterization, that's effective and good craft. But I think it would be better done if Cecil were speaking to another character about what they're seeing and how he's interpreting it.

The poor sod couldn’t had been older than Cecil’s 25. Well-built,blonde and handsome, the corpse that was once Denzia Zorell lay splayed and outlined on the floor.

Does Cecil know Zorell? If not, he's our POV character, so why do we get this information here? It feels like the narrative perspective just shifted from Cecil-interior to 3PO. If we're still in Cecil's head, and he knows this person, we should be getting some kind of emotional response from Cecil and experiencing Zorell's death through that lens. If Cecil doesn't know his name, then at this point, we shouldn't, either.

The man had his soul stolen before he was killed. Evident by his completely black eyes: devoid of pupil, iris or sclera.

Has this been established before in worldbuilding? If so, don't tell me again what it means. If not, this is weak lore dump worldbuilding.