See: How to write better (game) libraries

Core principles:

• Maximize portability.
• Be easy to build.
• Be easy to integrate.
• Be usable in as many scenarios as possible.

More specifically:

• Consider writing the library in C and add wrappers to other languages later.
• Consider providing an optional C++ wrapper.
• Try not to make the use of a build system mandatory.
• Ensure that people can easily compile the library from source.
• Minimize dependencies.
• Try not to allocate memory for the user.
• Be const correct.
• Always ask for the size of buffers (even for const char*)
• Try not to handle resources for user.
• Avoid using os-dependent functions (such as fopen) for maximum portability.
• Don't load OpenGL in the library if you depend on it.
• Provide examples.
• Use namespace prefixes.
• Avoid global variables if possible.
• Consider what error handling strategy you will use.
• Consider your distribution model.

Examples of good libraries:

• Sokol libraries (graphics, platform layer, audio, net): https://github.com/floooh/sokol
• BGFX (graphics): https://github.com/bkaradzic/bgfx
• STB libraries (asset loading, utilities): https://github.com/nothings/stb
• Cute Headers (graphics, net, io, utilities, etc): https://github.com/RandyGaul/cute_headers
• CGLTF (glTF loading): https://github.com/jkuhlmann/cgltf
• DR Libs (audio loading): https://github.com/mackron/dr_libs
• Mini audio (audio playback): https://github.com/dr-soft/miniaudio
• Par headers (graphics, utilities, etc): https://github.com/prideout/par
• Physac (physics): https://github.com/victorfisac/Physac
• Dear Imgui (ui): https://github.com/ocornut/imgui
• Soloud (audio): https://github.com/jarikomppa/soloud
• Meow Hash (non-crypto hash): https://github.com/cmuratori/meow_hash

• Single-file public-domain/open source libraries with minimal dependencies