Ringworld - Niven

Louis Wu has just turned 200 years old and he's becoming bored. He is something of an oddity in his (far future) society in that he is a risk taker. An alien from a race called the Puppeteers (herdlike, cowardly) has recruited him to investigate a mystery. The rest of the crew is a Kzin (think of a cross between Klingons and tigers) and a human who was 'bred for luck'.
The mystery is a ringlike structure, large enough to encircle a star. It has a radius similar to the Earth's orbit. The ring has a width of nearly 100,000 miles and the walls are a thousand miles high. The resultant room is roughly equivalent to 3 million times the area of the planet Earth. In other words, it's big. It's so big that it is too much for the brain to really comprehend. And it's the star of the book.
Don't get me wrong, Louis Wu and his companions are entertaining and the story is enjoyable. But the concepts really steal the show. The Ringworld is an absolute monster of an idea and it provides the broadest canvas ever. But Niven also plays with unbreakable materials. And true stasis fields. And teleportation. And a wonderful interplay between polar opposite aliens. Oh, and the most deadly sunflowers ever!
'Ringworld' is the hardest sci-fi so far in the Hugo collection. That means that the science creates something of a playground for the novelist. ('Soft' sci-fi deals with somewhat fuzzier subjects such as psychology, sociology and other areas of human behavior.) It still stands up, nearly 40 years later. It's a great book.