"Were you reading this book?"
Translation:Czytałeś tę książkę?
You mean why 'interesującą' ends with -ą but 'tę' with -ę? On one hand it's logical, because you have tĘ książkĘ. On the other hand, it's different from all the other situatiions, both tamtĄ książkę and forms of adjectives. I guess it's just something that you have to remember. Using "tą" is quite common, but still considered a grammar mistake, so we will not accept it.