W tej książce zakładka leży między stronami, na których autor opisuje spokojną kobietę z małego miasta.

Because the preposition w meaning in normally requires the locative case.

• ta książka = this book
• after w:
  • ta → tej
  • książka → książce

So:

• w tej książce = in this book

This is a very common pattern in Polish:

• w tym domu = in this house
• w tej szkole = in this school

Zakładka is the subject of the sentence, so it is in the nominative case.

The basic structure is:

• zakładka leży = the bookmark is lying / lies

So the sentence is built around the idea that the bookmark is located somewhere.

Also, zakładka most often means bookmark in this context.

Leży comes from leżeć, which means to lie or to be lying.

In Polish, it is very natural to use verbs like:

• leżeć = lie
• stać = stand
• wisieć = hang

to describe where something is physically located.

So:

• zakładka leży między stronami
  literally: the bookmark lies between the pages

In English, we often just say is, but Polish often prefers a more physical verb.

Because między here means between in a location sense, and with this meaning it takes the instrumental case.

• base form: strony = pages
• instrumental plural: stronami

So:

• między stronami = between the pages

A useful contrast:

• leży między stronami = it is lying between the pages → location
• wkładam ją między strony = I put it between the pages → movement toward a place, often with the accusative

So the form stronami matches the idea of a fixed location.

Because na których autor opisuje... is a relative clause, and in Polish relative clauses introduced by forms of który are normally separated by commas.

So the comma marks the part that means something like:

• on which the author describes...

This is standard Polish punctuation.

It is na których because the relative clause refers back to stronami (pages), and in Polish you normally say that something is written or described on pages:

• na stronach = on the pages

So the same preposition appears in the relative clause:

• na których = on which

As for grammar:

• których here is locative plural
• it matches the plural noun strony
• the case is required by na

So:

• na których autor opisuje...
  = on which the author describes...

Because kobietę is the direct object of opisuje (describes), so it must be in the accusative case.

Base form:

• spokojna kobieta = a calm woman

Accusative singular:

• spokojną kobietę

Both the adjective and the noun change because they must agree:

• spokojna → spokojną
• kobieta → kobietę

So:

• autor opisuje spokojną kobietę
  = the author describes a calm woman

Because the preposition z meaning from requires the genitive case.

Base form:

• małe miasto = a small town / a small city

After z:

• małego miasta

So:

• z małego miasta = from a small town

Again, both words change because the adjective must agree with the noun:

• małe → małego
• miasto → miasta

Opisuje is:

• 3rd person singular
• present tense
• imperfective
• from opisywać = to describe

It matches autor:

• autor opisuje = the author describes

The imperfective is natural here because it presents the action as an ongoing or general act happening on those pages. It is not focused on completion.

A perfective form would not fit as well here, because perfective present forms in Polish often have a future meaning.

Yes. Polish word order is much more flexible than English word order.

The original sentence begins with:

• W tej książce... = In this book...

That puts the location/topic first.

You could also say:

• Zakładka leży w tej książce między stronami...

The meaning stays very similar, but the emphasis changes slightly.

What usually matters most is:

• case endings
• keeping related parts close enough to be clear

For example, na których autor opisuje... stays near stronami because it describes which pages.

Polish has no articles, so it does not have separate words for a or the.

That means forms like:

• zakładka
• stronami
• kobietę
• miasta

can be interpreted as a or the, depending on context.

Specificity is shown by context, word order, or words like:

• ten / ta / to = this
• jakiś = some

In this sentence, tej książce is clearly specific because of tej = this, but zakładka itself could be understood as the bookmark or a bookmark depending on the wider context.