Here is a list of all documented files with brief descriptions:

src
checksum.h
cleanup.h
compression_wrapper.h
constants.h
createrepo_c.h
createrepo_shared.h
deltarpms.h
dumper_thread.h
error.h
helpers.h
koji.h
load_metadata.h
locate_metadata.h
mergerepo_c.h
misc.h
modifyrepo_shared.h
package.h
parsehdr.h
parsepkg.h
repomd.h
sqlite.h
threads.h
updateinfo.h
version.h
xml_dump.h
xml_file.h
xml_parser.h