Why is it 'den' instead of 'det'?
Both are correct, it depends on the gender of the noun being read.
"Dette er en bok. Jeg leser den." = "This is a book. I'm reading it."
"Dette er et ark. Jeg leser det." = "This is a piece of paper. I'm reading it."
I don't get what you mean by gender of the noun.
But can you tell me what gender do the things have, is "en" fem, masc or neither?
"De leser den" Is more correct because when referring to a book we use Den, however both is correct, but using Den is more correct than det.