(D7) – I’ve finished themy first book of theis year.

You're talking about the first book that you've finished this year.

Yesterday, I’ve finished reading the book titled "The Mysterious Affair at Styles" by Agatha Christie.

Put quotation marks on the title of the book.

I read it in English, and it’s been my first book ofby this amazing author.

Use a comma after the clause, I read it in English. Also, when we talk about books, we usually say it's by this author, not of this author in English.

I have to say I wasn’t prettyquite sure about reading this book, as the main reason lying behind it was to increase my vocabulary in English, and the book was written a long time ago.

The sentence you wrote was a run-on sentence, and I made it less lengthy by putting a comma after the clause, "As the main reason behind it was to increase my vocabulary in English."

Nevertheless, after finishing it, I’m very happy because the writing style and the vocabulary used in it isare very clear.

I put in a comma after the clause, after finishing it. I used the word are instead of is because you're referring to two things that you liked about this book, which are the writing style and the vocabulary.

The way the author presents the ideas is fantastic and pretty straightforward, which makes it easy to understand.

You didn't add anything about the book, which made it fascinating, so I dropped that word.

SWhile some words are out of fashion though, the outcome in general is good, and I totalwholeheartedly recommend it to English learners.

Some words are out of fashion though, the outcome in general is good is an incomplete sentence, so I added the transition word, while to make this sentence complete. Also, I used a more advanced word to describe how much you want others to read your book.

Anyway, the book is short, so if you've never read any book by this author but have the curiosity, this book might be a good first onchoice.

I changed the verb tense of the verb have to you have never read.

Yesterday, I’ve finished reading the book titled The Mysterious Affair at Styles by Agatha Christie.

I read it in English and it’s been my first book ofrom this amazing author.

I have to say that I wasn’t pretty sure about reading this book, as t. The main reason lying behind reading it was to increase my vocabulary in English and, but the book was written a long time ago.

Nevertheless, after finishing it, I’m very happy because the writing style and the vocabulary used in it isI were very clear.

The way the author presentsed the ideas iwas fantastic and pretty straightforward which makesde it easy to understand.

Some words are out of ld-fashion though, the outcome in general is good and I totallyed, yet I recommend it to English learners.

AnywayIn conclusion, the book is short so i. If you have never read any book by this author but have the curiosity,than this book might be a good first one.

I hope you can enjoy it as much as I did.

(D7) – I’ve finished theMy first book ofor theis year

Should probably be title case as well but the grammar is the key thing here

Yesterday, I’ve finished reading the book titled The Mysterious Affair at Styles by Agatha Christie.

small changes, it's a famous author and you also say it's a book next sentence so no need to say book again.

I read it in English and it’s been my first book ofby this amazing author.

I have to say that I wasn’t pretty sure about reading this book, as the main reason lying behind was to increase my vocabulary in English, and the book was written a long time ago.

Nevertheless, after finishing it I’m very happy because the writing style and the vocabulary used in it is very clear.

The way the author presents the ideas is fantastic and prettfairly straightforward which makes it easy to understand.

SAlthough some words are out of fashion though, the outcome in general i, the general outcome was good and I totally recommend it to English learners.

Anyway, the book is short so if you have never read any book by this author but have the curiosity, this book might be a good first one.

This isn't perfect but it's fairly natural.

I hope you canwill enjoy it as much as I did.

this is continuing from "if"