Documentation
User guide
The user guide (PDF) is a task-oriented guide to the API. It does not describe every detail of GLFW, but instead attempts to get you started in the areas of your interest.
Reference manual
The reference manual (PDF) describes every function and constant of the API, but isn't necessarily the best way to get started using GLFW.
Wiki
GLFW has a wiki at wiki.glfw.org.
Patch submitters guide
The patch submitters guide describes best practices for creating and submitting patches to GLFW. It is recommended reading for anyone who wishes to contribute to the project.