"Ringworld" by Larry Niven

The story opens with Louis Wu, a 200 year-old earthling living in the year 2850, enjoying his birthday by teleporting his way westerly around the globe from city to city to extend his birthday celebrations. Louis lives in a future where mankind has extended human life indefinitely through the use of "boosterspice", has colonised the surrounding stars and maintains civil relations with a number of other alien races. One of these races, the "Pierson's Puppeteers", have discovered a giant, ancient alien artefact while quietly migrating out of the galaxy (after discovering an astronomical phenomena in the centre of the milky-way that they believe will consume the galaxy in approximately 20,000 years). Nessus, a representative of the Puppeteers, recruits Louis to pilot an exploration mission to the artefact including two other crew members; Teela, an extremely lucky human female and "Speaker-to-animals", a "Kzin" (an large warrior cat-like alien).

The "artefact" is an enormous ribbon-ring structure approximately 600 million miles in circumference and a million miles wide which orbits a home star. Along the entire surface of the ring is a world, facing towards the sun, containing land, water, a bio-sphere and breathable atmosphere. After crash-landing on the surface of the ring (shot down by a meteor-defence system), the four explorers make their way across the ringworld trying to discover its original engineers, the purpose for building it and a way to re-launch their spacecraft home. The explorers encounter ruins and a technologically-regressed civilisation that religiously worships the ring. Eventually, after much exploring and encountering strange phenomena such floating cities and a field of sunflowers that fires laser beams (go figure), the explorers discover a giant mountain that turns out to be a hole punctured through the bottom of the ringworld, left when they crash-landed, and they use this to launch their spacecraft from the ringworld and return home.

Generally speaking, "Ringworld" is really not a great read. There is a lot that doesn't make sense, i.e. the notion of breeding genetically-associated "luck" by the Puppeteers, which was pretty unconvincing. The characters were generally boring; Teela was a horrible two-dimensional cardboard cut-out, and Nessus and Speaker were contradictory and confusing (to be fair, I guess there was perhaps a little bit of charm in their bizarre alien behaviour). That said, if you kind of skip over the ho-hum start of the novel and the sections on the ringworld that get a bit weird (I bring your attention again to laser-shooting sun-flowers), the concept of the ringworld itself is pretty amazing. It is the fascinating and sheer awe-inspiring scale that comes out of the notion of the ringworld itself that is the really interesting bit of the book. If you are a die-hard sf fan with time on your hands then, yes read this book, otherwise probably don't.