The World Inside (Robert Silverberg, 1971)

Urban Monad 116: A lofty spire a thousand stories high, where over 880,000 souls live out their perfectly regulated lives in peace and plenty. But inside their glorious world are a few who dare to doubt and dream: Aurea Holston - a beautiful young bride who fears leaving the only world she's ever known. Dillon Chrimes - cosmos group pop star, who becomes one of the urbmon in an orgiastic, mind-shattering trip. Jason Quevedo - historian, who gets his kicks from the perverse savagery of an earlier age. Siegmund Kluver - virile young man-on-the-way-up, who sees the nightmare behind the urbmon's shining facade. And Michael Statler - who dares to escape...

Two similar books dealing with overpopulation:
Stand on Zanzibar by John Brunner
334 by Thomas Disch