Die Autorin, deren Roman ich lese, schreibt sehr spannend.

deren is the relative-word equivalent of whose.

In this sentence, it links the relative clause back to die Autorin and shows possession:

• die Autorin = the person being talked about
• deren Roman = that person’s novel

So deren does not mean her in the ordinary personal-pronoun sense. It is specifically the form used inside a relative clause to mean whose.

Because deren refers back to die Autorin, which is feminine singular.

For possessive relative pronouns in German:

• dessen = whose for a masculine or neuter noun
• deren = whose for a feminine noun or a plural noun

So:

• der Autor, dessen Roman ... = the male author whose novel ...
• die Autorin, deren Roman ... = the female author whose novel ...

The form depends on the noun being referred back to, not on Roman.

Because deren already functions like a determiner, similar to ihr, sein, mein, etc.

So German says:

• deren Roman
• ihr Roman
• sein Roman

not:

• deren der Roman

You can think of deren as already occupying the article/determiner slot in the noun phrase.

Roman is in the accusative, because it is the direct object of lese.

Inside the relative clause:

• ich = subject
• lese = verb
• deren Roman = thing being read, so direct object

So grammatically it is accusative masculine.

Why does it look the same as nominative? Because with a masculine singular noun by itself, the noun often does not visibly change form. If there were an adjective, you would see it more clearly:

• deren neuen Roman ich lese

Here neuen shows the accusative masculine more obviously.

Because deren Roman ich lese is a relative clause, and relative clauses are subordinate clauses in German.

In German subordinate clauses, the finite verb usually goes to the end.

So the order is:

• deren Roman
• ich
• lese

That is completely normal for German relative clauses.

Because it is a relative clause, and German normally sets relative clauses off with commas.

So:

• Die Autorin, deren Roman ich lese, schreibt sehr spannend.

The commas mark the inserted clause that gives extra information about die Autorin.

This is stricter in German than in English: in German, these commas are not optional here.

Because German main clauses follow the verb-second rule.

The finite verb in a main clause must come in the second position. Here, the entire subject phrase counts as the first element:

• Die Autorin, deren Roman ich lese = position 1
• schreibt = position 2

Even though that first element is long, it still counts as just one unit for word-order purposes.

Because spannend is being used adverbially here: it describes how she writes.

After verbs like schreiben, German often uses an adjective in adverbial function, and in that use it does not take an ending:

• Sie schreibt spannend.
• Er spricht schnell.
• Das klingt gut.

You only add adjective endings when the adjective comes before a noun:

• ein spannender Roman
• eine spannende Autorin

But here there is no noun after spannend, so no adjective ending appears.

Not in standard German if you want a proper relative clause.

You need deren because the clause is attached to die Autorin as a relative clause:

• Die Autorin, deren Roman ich lese, ...

Using ihr would not work the same way, because ihr is a possessive determiner, not a relative pronoun.

So:

• deren = correct for whose
• ihr = means her, but not as a relative-linking word in this structure

In sentences like this, yes, that is usually the best English equivalent.

But grammatically, it is more precise to think of deren as a genitive relative pronoun. That matters because it behaves according to German grammar, not English grammar.

A useful way to remember it is:

• deren / dessen = possessive relative forms
• they connect a relative clause
• they show ownership or association

So in practice, whose is usually the right meaning, but the German form follows its own case-and-gender system.