Моя подруга сказала, что этот детектив стоит перечитать из-за деталей, которые сначала кажутся неважными.

Because the subject is подруга, which is a feminine noun.

• моя подруга = my female friend
• In the past tense, Russian verbs agree with the subject in gender:
  • сказал = he said
  • сказала = she said
  • сказало = it said
  • сказали = they said

So Моя подруга сказала means My friend said, with both the pronoun and the verb showing that the friend is female.

It can mean either, depending on context.

• подруга = a female friend
• In some contexts, it can also mean girlfriend

Here, since the sentence is neutral and about recommending a book, it most naturally means female friend.

This is a common point for English speakers because English separates friend and girlfriend, while Russian sometimes relies on context.

Что introduces a subordinate clause and usually means that in sentences like this.

So:

• Моя подруга сказала = My friend said
• что этот детектив стоит перечитать... = that this detective novel is worth rereading...

Russian often uses что where English may use that, even in places where English might omit it.

Here it means a detective story/novel, not a detective person.

That is clear because of the verb перечитать = to reread. You reread a book, not a person.

So этот детектив here means something like:

• this detective novel
• this mystery
• this detective story

Russian детектив can refer to the genre as well as a work in that genre.

Naturally, it means is worth rereading.

The pattern is:

• стоит + infinitive = it is worth doing something

Examples:

• Это стоит посмотреть. = This is worth watching.
• Книгу стоит прочитать. = The book is worth reading.
• Этот детектив стоит перечитать. = This detective novel is worth rereading.

So don’t translate стоит too literally as stands here. It is an idiomatic construction.

Because перечитать means to read again / reread, while читать just means to read.

The prefix пере- often adds the idea of again or over.

• читать = to read
• прочитать = to read through / finish reading
• перечитать = to reread

In this sentence, the idea is that the book deserves a second reading because some details matter more than they first seem.

Also, perfective перечитать fits well because it refers to one complete rereading.

Because из-за requires the genitive case.

• из-за = because of / due to
• детали is the nominative plural
• деталей is the genitive plural

So:

• из-за деталей = because of the details

A useful thing to know: из-за often has a slightly negative flavor, like because of in because of all these problems, but in real usage it can also be neutral, as here.

Которые is a relative pronoun meaning which or that.

It refers back to деталей = details, but its case is decided by its role inside its own clause, not by the case of деталей.

Here the clause is:

• которые сначала кажутся неважными = which at first seem unimportant

Inside that clause, которые is the subject of кажутся, so it is in the nominative plural.

That is why it is которые, not a genitive form.

It is plural because the subject is plural: которые = details.

• singular: кажется = seems
• plural: кажутся = seem

It is reflexive because the dictionary form is казаться, which means to seem / appear.

Examples:

• Это кажется странным. = This seems strange.
• Они кажутся знакомыми. = They seem familiar.

So которые сначала кажутся неважными means which at first seem unimportant.

Because after казаться, the adjective is commonly put in the instrumental case.

So:

• кажутся неважными = seem unimportant
• неважными is instrumental plural
• it agrees with детали in number, but takes instrumental because of the construction

This is a very common pattern:

• Он кажется усталым. = He seems tired.
• Она казалась счастливой. = She seemed happy.
• Они кажутся неважными. = They seem unimportant.

So English speakers often need to remember: after казаться, predicate adjectives are often instrumental.

Сначала means at first or initially.

In this sentence:

• которые сначала кажутся неважными = which at first seem unimportant

It shows that the first impression is misleading. Later, those details turn out to matter.

It can move around somewhat in the sentence, but its current position is very natural.

Yes, Russian word order is flexible, but this version is natural and clear.

The sentence is structured like this:

• Моя подруга сказала = main clause
• что этот детектив стоит перечитать = content of what she said
• из-за деталей = reason
• которые сначала кажутся неважными = description of those details

This order gives a smooth flow:

1. who said it
2. what was said
3. why it is worth rereading
4. what kind of details they are

You could rearrange parts for emphasis, but this version sounds neutral and standard.

Yes. Russian often omits demonstratives when they are not needed.

• этот детектив = this detective novel
• детектив = a/the detective novel, depending on context

Using этот makes it more specific, as if the speaker has a particular book in mind.

So the sentence with этот suggests something like:

• this particular mystery novel
• this one

Yes—this sentence is a good example of several common Russian patterns at once:

• past tense agrees in gender: сказала
• что introduces a that-clause
• стоит + infinitive means is worth doing
• из-за takes the genitive: деталей
• которые is chosen by its role in the relative clause
• казаться often takes an instrumental adjective: неважными

So even though the sentence may look long, it is built from very standard and useful pieces.