"They are reading it."

[VitGryfny]

Why is it 'den' instead of 'det'?

[fveldig]

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."

[Colin846228]

I don't get what you mean by gender of the noun.

[fveldig]

Grammatical gender

[anoukie1911]

But can you tell me what gender do the things have, is "en" fem, masc or neither?

[Chrispoot]

"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.