Implementing a compliant C library requires knowledge across many areas. Here you can find relevant specifications that'll help you in your quest. If you know of other things that could be useful, please let us know!
These are standards to be aware of; generally uClibc aims to be compliant with the latest POSIX standard.
These are documents related to the common ELF format.
These are architecture-specific ELF documents.
These are general architecture related reference manuals.