‘This is the book of which I told you.’ Here the word ‘which’ is used as -

Relative pronouns introduce relative clauses. The most common relative pronouns are who, whom, whose, which, that. The relative pronoun we use depends on what we are referring to and the type of relative clause. We use which in relative clauses to refer to animals and to things. We use it to introduce defining and non-defining relative clauses. We always use which to introduce relative clauses when they refer to a whole sentence or clause. [source: dictionary.cambridge.org]