In English there are three different relative pronouns

Antonio Buero Vallejo, who is the most famous Spanish dramatist of post war time, died in 2000.
The book, which was written by Antonio Buero Vallejo, was very interesting to read.
The film, which adapted the book by Antonio Buero Valejo, was done very well.

The German only has two types of relative pronouns. The first one, der, die, das etc. we have already presented. These relative pronouns are identical to the demonstrative pronouns der, die, das. The other type is welcher, welche, welche is identical to the interrogative pronouns welcher, welche, welches. There is actually no difference between these two types, just that der, die, das is used much more frequently than welcher, welche, welches.

The declination of welcher, welche, welches.     masculine feminine neutral   Singular         Nominative welcher welche welches   Genitive wessen (not used) wessen (not used) wessen (not used)   Dative welchem welcher welchem   Accusative welchen welche welches   Plural         Nominative welche welche welche   Genitive wessen (not used) wessen (not used) wessen (not used)   Dative welchen welchen welchen   Akkusativ welche welche welche

Let's have a look at an example   we have two phrases     Das Buch ist langweilig. = The book is boring Ich lese das Buch. = I'm reading the book.   These two phrases shall be combined to one phrase The book, that I'm reading, is boring.

The main phrase is Das Buch ist langweilig. The object the relative pronoun is referring to is das Buch. Das Buch is singular neutral. In the main phrase das Buch is in Nominative, but that is not too interesting. That what's interesting is the declination of the relative phrase. In the relative clause das Buch is in Accusative -declination.

Ich lese das Buch. Was (oder wen) lese ich? Das Buch. = I'm reading the book. What do I read? The book.

So we need the relative pronoun in neutral singular and in Accusative -declination and this is - according to our table, welches. The phrase is:

Das Buch, welches ich lese, ist langweilig.