can you please help me to understand how the relative pronouns which and that are considered to be objects in the example below.Thank you.
In Betty Azar's Chartbook (a reference guide to grammar p 82) the following sentence is given:
The books which/that I bought were expensive.
The subject-verb pair = I bought
However, I don't understand why which (or that) are given as the object of the sentence given that the object of a verb is defined as being the thing/person that receives the action of the verb.