It’s part of Windows Installer XML (WiX) an open source project formerly from Microsoft but since transferred to the Outercurve Foundation. It can be found on CodePlex. 3.7 is the latest release.
This interop assembly is part of Deployment Tools Foundation (DTF) and you’ll find an SDK help file installed in the start menu. The actual assembly will be found in C:\Program Files (x86)\WiX Toolset v3.7\SDK.
Update 5/10/2023-
As part of WiX Toolset’s migration to Nuget and ProjectStyle SDKs, this assembly has been renamed to WixToolset.Dtf.WindowsInstaller and is available via Nuget.
https://www.nuget.org/packages/WixToolset.Dtf.WindowsInstaller