Sentence:  There are moments when he seems to be writing something on the order of Jorge Luis Borges's "Tlön, Uqbar, Orbis Tertius"—an amazing fragment about the <WSD>alternate universe</WSD> where every book contains its counterbook, where things become duplicated and "also tend to become effaced and lose their details when they are forgotten." 
Meaning of the word marked with <WSD> tags:  An imaginary realm, often a variant form of the real world, depicting a different way events could have unfolded or the universe could have functioned.