It's about the project of colonizing and terraforming Mars. The book follows the first Hundred - the first group of colonists (extremely bright and accomplished scientists & engineers) who land on the planet and proceed to build a world with their hands from scratch. A goodly third of the book is dedicated to landscape description of Mars, you feel like you actually learn geography (and bits of other science). It's done in a very realistic way. Honestly, I'd skim the landscape descriptions.

The speed picks up - more people travel to Mars, establish settlements and towns and their own cultures, and politics gets involved - the question of self-government, relations to Earth & multi-national corporations, how extreme the terraforming process should be, and on. The books walk through every sociopolitical and physical aspect of colonization and all the problems that arise.

So far it sounds dull as hell, I know. However, the story follows individuals with their fiery temperaments, mad scientist plans (they do some cool scifi stuff, like invent immortality - with its consequences), love & betrayals. You become completely wrapped up in that world, and when characters die, it's heartbreaking. At the big climax at the end of book 1, I actually shed a tear.

What's appealing about the book is the essense of the appeal of sf/f in general - the fantasy of political agency (in the words of Lois McMaster Bujold). You get a sense of power, like anything is possible. The juicy parts are when we see how a handful of individuals (not kings or prophets, but just really smart scientists) can determine the fate of the world, one step at a time. And then you can see the world transforming. The series follows Mars' physical terraforming process.

I thought it was the coolest thing ever, and actually switched colleges so I could study engineering under the influence of this book.