Describing the book The Orange Girl part. I

The Orange Girl is a book written by a Norwegian philosopher, J. Gaarder.
The plot  it is about a 13-year-old teenager, Robert, who finds a letter  of from his dead father. His father had an illness, and when the main character was one year old, he died. Before this tragic moment, his father had been writing a letter to Robert, having in mind that this letter would be read in the future by his son, but without knowing when.

Robert's father expressed to him his passion for the Universe, as well as all the concerns he had about human existence. In this letter it is also written who is The Orange Girl and why is she so important in Robert's life (my suggestion:  "This letter also explains who The Orange Girl is and why she is so important in Robert's life). By reading this letter, Robert will realise that he has more things in common with his father than he would ever have imagined.

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Overall, your grammar is very good.   I only had to fix a few misspellings and make some other small corrections.

I changed the book title to "The Orange Girl" because that is the title the publisher used when they published this novel in English

When a person's age is used as an adjective, we write "___ -year-old" instead of "___ years old."    For example, my son is sixteen years old, but if my sentences use his age as an adjective, I should write, "My sixteen-year-old son..."  or  "He is a sixteen-year-old boy."

Numbers up to ten are usually spelled out as words.  For numbers bigger than ten, it is usually the choice of the writer whether to use numerals or spell them out as words.

Advice given from the perspective of someone living in the United States.  It is possible that some conventions are different in other English-speaking countries.