The Stacks project is an open source text book and reference work. Its topic is algebraic stacks and the algebraic geometry that is needed to define them. The current version of the entire book can always be found here. You can also browse the chapters online by going to this page. You can freely download absolutely any of the files of the stacks project from this page.
The stacks project is a collaborative effort, but not a wiki. Its development model is more like that of an open source software project. In particular there is a maintainer who decides which changes go into the project, etc. The current maintainer is A.J. de Jong.
The stacks project is a book in progress. You see the recent changes to the project at this link where you can find the complete development history.
How can you reference lemmas, propositions, and theorems in a book in progress where the numbering changes over time? To do this you refer to a mathematical result by its tag. The tags system is explained here and here.
How can you contribute? Here are some suggestions:
- You can leave comments on this blog if you want to give feedback in an informal manner.
- Use the stacks project by reading it! As you read you will encounter typos, small errors, places where the exposition is confusing, notation you don’t like, etc. Just email anything you find to firstname.lastname@example.org.
- Download the tex and pdf file of a chapter, and while you are reading correct any typos, or mathematical errors you see. At the end simply email the new tex file to email@example.com.
- You can take a look at the todo-list to get some suggestions for things that need to be done.
More global information can be found in this blog post.