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

jxl
cms.h
cms_interface.h	Interface to allow the injection of different color management systems (CMSes, also called color management modules, or CMMs) in JPEG XL
codestream_header.h	Definitions of structs and enums for the metadata from the JPEG XL codestream headers (signature, metadata, preview dimensions, ...), excluding color encoding which is in color_encoding.h
color_encoding.h	Color Encoding definitions used by JPEG XL. All CIE units are for the standard 1931 2 degree observer
compressed_icc.h	Utility functions to compress and decompress ICC streams
decode.h	Decoding API for JPEG XL
decode_cxx.h	C++ header-only helper for decode.h
encode.h	Encoding API for JPEG XL
encode_cxx.h	C++ header-only helper for encode.h
gain_map.h	Utility functions to manipulate jhgm (gain map) boxes
memory_manager.h	Abstraction functions used by JPEG XL to allocate memory
parallel_runner.h
resizable_parallel_runner.h	Implementation using std::thread of a resizeable JxlParallelRunner
resizable_parallel_runner_cxx.h	C++ header-only helper for resizable_parallel_runner.h
stats.h	API to collect various statistics from JXL encoder
thread_parallel_runner.h	Implementation using std::thread of a JxlParallelRunner
thread_parallel_runner_cxx.h	C++ header-only helper for thread_parallel_runner.h
types.h	Data types for the JPEG XL API, for both encoding and decoding