Ringworld by Larry Niven
Winner of Hugo and Nebula awards.
This is a classic SF novel, and one that tends to come to mind when Larry Niven's name is mentioned.
The focus of the book is "Ringworld" - a vast artificial habitat. Long ago, some race built a structure that is a million-mile wide strip circling all the way around a star at the distance similar to Earth's orbit from the Sun. The surface area is equal to millions of planets. There are miles-high walls along the edges of the strip to keep in the atmosphere. It's an awe-inspiring thing - sort of one the Seven Wonders Of The Galaxy. (Too bad it's not really feasible.)
Niven introduces us here to the Pierson's Puppeteers extraterrestrial species. They are a race that is "very cautious", well they're practically afraid of their own shadows. A huge wave of radiation is coming from the center of the galaxy and is due to arrive in 20,000 years, and they've already packed their bags, converted their planets into spaceships and headed out to safer regions.
One of the Puppeteers, Nessus, is "insane" - he isn't as fearful as "normal" Puppeteers. Since he is willing to do things others of his kind would not, he has been sent on a mission. He gathers a human adventurer Louis Wu, a kzin warrior Speaker-To-Animals, and a young girl. He takes them to take a look at Ringworld. When they get there, Ringworld's defense system shoots down their spaceship.
As with all-to-many other SF stories by now, the society of Ringworld's inhabitants isn't what it used to be. It's no longer a hi-tech place where you can find a good starship repair shop. The group spends a good part of the book going to places on Ringworld, finding out about it and its downfall, and trying to find a way to get back into space.
All along Nessus is either uninformative or misleading about what his full agenda is. The unraveling of this is also an important storyline. There is also time spent considering what caused the Ringworld civilization to fall. (More details of who built Ringworld and what happened are left to the sequels.)
The inclusion of Puppeteers and kzin also give a bit of a look at aliens.