The Ringworld Engineers

Boring. It comes across with all the worst character tropes, has a poorly driven plot, side-quests to meet the locals that come across as banal and seemingly provide no character motivation (Louis kills them all anyway! With hardly a second thought!) and−worst of all−has this weird affection for inter-species sex that provides absolutely no benefit in the book. For god's sake Niven, if you're gonna put sex into a book, don't be shy about it. It's your own damn book!