Sadly, there is only one decent bookshop in Port Moresby: the UPNG Bookshop and even that shop stocks mostly coursework materials and academic texts. Top-end hotels have a few overpriced books and there are a couple of duty-free shops at the airport (past Immigration) that have some decent books on PNG. Make sure you bring your own reading material because you can't even buy a trashy airport novel in Port Moresby.
UPNG Bookshop
Port Moresby