I finished reading themy book three days ago. I finished reading my book three days ago.

"The" doesn't really work here as you haven't established WHICH book it is yet. Using "my" will establish this.

I wrote about the half of the details in a diaprevious diary entry. I wrote about half of the details in a previous diary entry.

A diary = the whole book A diary entry = just 1 singular entry OR: I wrote about the details of the first half of the book in a previous diary entry.

In those three days, I searched how I can say the words that I don't know how to say in English. In those three days, I searched how I can say the words that I don't know how to say in English.

OR: how TO say the words I don't know

Because the book had some explanationparts that are difficult to diescribe in English. Because the book had some parts that are difficult to describe in English.

Explanation wouldn't work here. You could say descriptions, but you already use the word "describe" later in the same sentence so that would sound too repetitive.

I diescribed in a diary thatprevious diary entry that in the book there is a handyman who speaks like a machine, with a face that never changes. I described in a previous diary entry that in the book there is a handyman who speaks like a machine, with a face that never changes.

However, the clients can't see him again, because he doesn't tell them his name and he changes thehis phone numbers every time. However, the clients can't see him again, because he doesn't tell them his name and he changes his phone number every time.

Tell needs an object afterwards --> I will tell her, I told them, he tells them, he told me, etc.

There is also a man who workeds in the main police station in Athe city and had used the handyman before. There is also a man who works in the main police station in the city and had used the handyman before.

"Worked" should be "works" here if you are going to continue the story in the next sentence by saying he later quits. Him quitting later implies he is still working there before then.

TAs the story continues, as the man quits his job and heldstarts his own private detective officebusiness. As the story continues, the man quits his job and starts his own private detective business.

You only "hold" office if you are a politician. Otherwise you can work in an office or (even better here as he is the boss) start a business.

BecauseHe did this after the hanother man came asdyman was promoted to the highest position ato the main police office in Athe city. He did this after the handyman was promoted to the highest position at the main police office in the city.

No need to capitalise "A" here. The would work better anyway. "Another" OR "the other", NEVER "the another" I'm not sure if you mean the handyman or the detective here.

PThis meant that all the city's police officers were influencesd by him and couldn't deal with cases properly, andso the city was filled with many unsolved murder cases. This meant that all the city's police officers were influenced by him and couldn't deal with cases properly, so the city was filled with many unsolved murders.

So that it gained people who is in veryThis attracted the attention of the people in the highest positions inof the city center's attention. This attracted the attention of the people in the highest positions of the city.

Attention is often "attracted"

HeThe handyman felt that if he was honest, he would be blamed and be fired. The handyman felt that if he was honest, he would be blamed and fired.

It's a bit confusing which man you are referring to - best to call them by nicknames, such as the handyman and the detective, to make it clearer.

Because he killed people to got plenty offor money. Because he killed people for money.

OSo, one day he went to the hospital and had a doctor surgically implant a poison chip into his arm. So, one day he went to the hospital and had a doctor surgically implant a poison chip into his arm.

He met thea lawyer that known as heew he was involves tod in a lot of wrongdoings, when he investigated tcrimes as thehis job. He met a lawyer that knew he was involved in a lot of wrongdoings, when he investigated crimes as his job.

When the lawyer investigated crimes? It's a bit unclear here.

And They joined forces to achieve each other's perposegoals by sharing informations. They joined forces to achieve each other's goals by sharing information.

Information is never plural. So no "S" at the end. Goals would sound a lot better than purpose. Someone's purpose is usually a lot more philsophical, as in the one singular purpose of their entire life, or their reason for living.