From a Win32 .NET library (One that targets desktop apps whether Console, WinForms or WPF) you can add two references and take advantage of (most) UWP APIs.
The first is a .NET library which handles interop with the UWP / WinRT APIs:-
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETCore\v4.5\System.Runtime.WindowsRuntime.dll
The second is the metadata for the UWP APIs themselves:-
C:\Program Files (x86)\Windows Kits\10\UnionMetadata\Windows.winmd
This describes the v1.0 versions of the API Contracts. These are expanded with each Windows 10 version and so you may find you want to target newer APIs and they are not available here. In a UWP project you can easily toggle the minimum target version of Windows 10 and this defines the API surface available to you. No such option exists for this kind of interop so instead you can switch the Windows.winmd reference for an alternative version. These are installed into subfolders of the above folder with the build number of each installed SDK. The latest released version for the April 2018 update is in 10.0.17134.0. As with UWP development you need to choose your API level carefully as it will affect which machines you can deploy to. For a library developer you need to ensure you document the minimum version you support to avoid any surprises.