Ten czytelnik lubi krótkie powieści, ale ja wolę książki, w których każdy akapit jest dłuższy.

Ten means this and agrees with czytelnik (reader) in gender, number, and case.

• ten czytelnik = this reader
• czytelnik by itself = reader / the reader / a reader, depending on context

Polish does not have articles like a or the, so words like ten are often used when you want to point someone out more clearly.

Because lubić (to like) takes the direct object in the accusative case.

Here, powieści is the object of lubi:

• Ten czytelnik lubi krótkie powieści = This reader likes short novels

For plural non-masculine-personal nouns like powieści, the accusative plural looks the same as the nominative plural:

• nominative: krótkie powieści
• accusative: krótkie powieści

So even though the form looks unchanged, it is functioning as the accusative here.

Powieści is the plural of powieść (novel).
Krótkie is the adjective short, and it agrees with powieści in:

• number: plural
• gender/class: non-masculine-personal
• case: accusative plural here

So:

• singular: krótka powieść = a short novel
• plural: krótkie powieści = short novels

No, ja is not strictly necessary, because the verb form wolę already tells you the subject is I.

• wolę = I prefer
• ja wolę = I prefer, with extra emphasis

Including ja often adds contrast or emphasis. In this sentence, it helps express:

• This reader likes short novels, but I prefer...

So ja is used here for contrast with ten czytelnik.

Wolę is the 1st person singular present tense of woleć (to prefer).

Conjugation:

• (ja) wolę = I prefer
• (ty) wolisz = you prefer
• (on/ona/ono) woli = he/she/it prefers
• (my) wolimy = we prefer
• (wy) wolicie = you prefer
• (oni/one) wolą = they prefer

So:

• ja wolę książki = I prefer books

Because woleć also takes a direct object in the accusative case.

• książka = book
• książki = books

Here książki is accusative plural. For this kind of plural noun, the accusative form is the same as the nominative form:

• nominative plural: książki
• accusative plural: książki

So:

• wolę książki = I prefer books

W których means in which.

It refers back to książki (books) and introduces a relative clause:

• książki, w których... = books in which...

Why których? Because the preposition w usually requires the locative case when it means in.

So:

• które = which (nominative/accusative type forms, depending on context)
• w których = in which (locative plural)

Since książki is plural, the relative pronoun must also be plural:

• singular: w której = in which
• plural: w których = in which

Both commas are required by standard Polish punctuation.

1. Before ale
   Ale means but, and coordinating conjunctions like this are normally preceded by a comma:

   • ..., ale ...

2. Before w których
   This introduces a relative clause:

   • książki, w których każdy akapit jest dłuższy
   • books in which every paragraph is longer

In Polish, relative clauses are set off by commas.

Because każdy akapit is the subject of the relative clause:

• każdy = every / each
• akapit = paragraph

Together:

• każdy akapit = every paragraph

As the subject, it is in the nominative singular.

Każdy agrees with akapit because akapit is:

• masculine
• singular
• nominative

So the clause literally is:

• every paragraph is longer

Dłuższy is the comparative form of długi (long):

• długi = long
• dłuższy = longer

So:

• jest dłuższy = is longer

Why comparative?
Because the sentence is saying that in the books the speaker prefers, every paragraph is longer.

Why not długi?

• jest długi = is long
• jest dłuższy = is longer

Why not długo? Because długo is an adverb meaning for a long time / long, and here we need an adjective describing akapit (paragraph), which is a noun.

Good question. In this sentence, the comparison is left implicit.

Dłuższy can mean:

• longer than something understood from context

Here the idea is something like:

• longer than the paragraphs in the short novels
• longer than usual
• simply longer in the kind of books I prefer

Polish, like English, does not always need to state the second half of a comparison if it is clear from context.

If you wanted to make it explicit, you could say something like:

• ...w których każdy akapit jest dłuższy niż w tamtych powieściach
  = ...in which every paragraph is longer than in those novels

Yes, Polish word order is fairly flexible, though not completely free. The original sentence is natural and neutral:

• Ten czytelnik lubi krótkie powieści, ale ja wolę książki, w których każdy akapit jest dłuższy.

You could rearrange parts for emphasis, for example:

• Ja wolę książki, w których każdy akapit jest dłuższy, ale ten czytelnik lubi krótkie powieści.

That still works, but the emphasis changes.

In Polish, word order often helps show:

• what is already known
• what is new information
• what is being contrasted or emphasized

So the original version is a good standard model.

It can absolutely be used with books.

The preposition w often means in, and in English we also say:

• books in which...

So:

• książki, w których każdy akapit jest dłuższy means
• books in which every paragraph is longer

This is not about physical location only. It can refer to something contained within a text, book, document, story, and so on.

Other similar examples:

• film, w którym gra mój brat = a film in which my brother acts
• miasto, w którym mieszkam = the city in which I live
• artykuł, w którym o tym pisano = the article in which they wrote about it

It has two main parts joined by ale (but):

1. Ten czytelnik lubi krótkie powieści
   = This reader likes short novels

2. ja wolę książki, w których każdy akapit jest dłuższy
   = I prefer books in which every paragraph is longer

Inside the second part, there is a relative clause:

• w których każdy akapit jest dłuższy
• in which every paragraph is longer

So the structure is:

• main clause 1
• ale
• main clause 2
• relative clause modifying książki

This kind of sentence is very common in Polish.