The glibc manual is available online here. The manual contains the canonical API descriptions.
The glibc project does not maintain any man-pages; instead other projects document the glibc APIs as man-pages. The largest such collection is the Linux man-pages project. The glibc community keeps close ties with the Linux man-pages project, and many developers contribute to both sources of documentation. While the glibc manual remains the canonical source for API descriptions, the man-pages are an excellent reference. Every effort is made to keep the documentation harmonized between both projects.
The glibc wiki contains a lot of important information. Please see "Editing the wiki" for instructions on getting an account and editing. Please feel free to add to the community knowledge, feature wishlists, feature designs, debugging tips, or developer notes.
The glibc FAQ is available on the wiki here: FAQ.