This short book (roughly 100 pages) gives a clear exposition of the basic elements of axiomatic general equilibrium analysis. The first chapter introduces all (sic!) mathematics used in this book, mainly some topology of euclidean space and basic facts about convex sets. In principle only knowledge about counting is necessary, but some "mathematical maturity" is clearly required. I would advise the reader to learn the relevant topology elsewhere ("Introduction to Analysis" by M. Rosenlicht suffices) and use the first chapter only for reference.
The main text covers the Arrow-Debreu-McKenzie-model and its interpretation, proves its logcial consistency (existence) and investigates its efficiency properties. The formal model is clearly distinguished from its interpretation, which allows Debreu to introduce uncertainty in the model by a simple reinterpretation of the commodity space. The whole approach is axiomatic, which wasn't that usual when Debreu wrote the book in 1959. This book has changed the standards of mathematical rigor in economic theory.
This book is still used as a reference and deserves a place on every economic theorists bookshelf.