I forgot the name of the mystery novel. I only remember some fragments. Can you recommend it?

He probably remembered that the male protagonist's father had gone missing. He ran to find the huge secret left behind by his father. There were others who were also looking for it and competing with each other.
Memories:
1. They went to a place where there was a strange picture. The person who saw the picture would have a stooped figure in his eyes and would soon die.
2. A strange monkey-like creature they encountered during the search. It was called a shadow in the text. Shadows could pass their memories to other shadows. Humans captured shadows and took out their brains to dry them into ashes. They added them to candles and lit them. When they inhaled them, they would see the memories of the shadows when they were alive.
3. There was a black cube that could duplicate a person into two, but only one person could survive, so the clone would come and kill the original.
4. There was a strong-willed man who got eternal life by copying himself with a black square. He discovered this secret, which would lead to a great disaster if it was revealed. Therefore, he had been persuading various leaders to build a talisman to suppress this secret. The Great Wall was a part of this talisman.
Then, he remembered that the back view of the monkey in 1 was the back view of the monkey in 2. The back view with a shadow in the pupil would die. After the male protagonist's friend saw the painting, the back view appeared in his eyes.

Answer 1

I don't think it'll necessarily meet your needs, but I'll try my best to recommend a novel that's most relevant to your description."Mystery Dreams" is a mystery detective and horror novel written by Mystery House. The general plot of this novel is as follows:

I didn't know if this was real or a dream!
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"Strange Murder Record" was a mystery detective novel written by the author Yao Ju. The general plot of this novel was:

In a single thought, a single step was enough to move people's hearts. In front of the abyss, take a step, eternal damnation; On the way back, look back quickly, as before.
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"The Strange Case" was a suspense-mystery detective novel written by the author, Wu Jian Wen. The general plot of this novel was:

Truth and falsehood, truth and falsehood, inaction, existence and nothingness, searching for the truth in the fog. Little brothers, this is my first time writing a novel. I'm a newbie, O(headquarter)O.
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"The Strange Case" was a suspense-mystery detective novel written by the author, Young Master Gui. The general plot of this novel was:

Have you ever seen a patient with a twisted mind? Have you ever seen a music academy that was rumored to have a lullaby of death? Have you ever seen a living dead assassin resurrected at a banquet? I do! I am He Sheng, a legendary coroner. Take you straight to the scene of a strange and strange unsolved case. The justice of heaven is long and there are no ghosts in the world.
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"Records of the Underworld" was a mystery detective and adventure novel written by Chen Xiaotai. The general plot of the novel was:

A simple version of a toiling life!
[Suspense version: A Heavenly Palace Realm, a past life, countless supernatural events, and many good stories!]
Impression version: Small supernatural, small magic, small suspense, small scheme, small joy, small love, and a little bit of a small mistake!
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"Miss Detective of Criminal Investigation" was a mystery detective novel written by the author, Jianli. The general plot of the novel was:

Looking for the truth in the fog,
Looking for the murderer in the midst of danger,
Live for the truth, die for the truth.
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"Mystery and Fear" was a mystery detective and adventure novel written by the author. The general plot of the novel was:

She was destined to be cursed. Why did death always happen to her? Was he a ghost from the darkness? How could a person come back to life? Time and time again, encounters and struggles, was it the beginning of beauty or the end of sin? When the layers of fog were lifted, he was actually…Before reading, there was no truth. Are you ready?
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Return to the Scene of a Murder was a suspense-mystery detective novel written by Lin Family Number 37. The general plot of the novel was:

Zhang Xingcheng realized that if there was really another hand behind all this, then this person seemed to be coming for him! However, the missing memories in his mind could not make Zhang Xingcheng remember where he had seen this symbol or smelled that fragrance! What was the other party trying to express?
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Soul Guest House was a mystery detective novel written by a rabbit head. The general plot of the novel was:

After many years of hard work, the ghost emissary Chu Qi realized that he was a ghost with a story, and a ghost with a family…
Can you return my memories to me?
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"Anecdote" was a suspense novel written by the author, Zero Source. The general plot of the novel was:

I don't know if you've ever heard the old people tell you over and over again about the miserable stories of demons and ghosts. You only think of them as stories, but you don't know that these are right beside you…