Steamworks.NET is a C# Wrapper for Valve's Steamworks API, it can be used either with Unity or your C# based Application.
Steamworks.NET was designed to be as close as possible to the original C++ API, as such the documentation provided from Valve largely covers usage of Steamworks.NET. Niceties and C# Idioms can be easily implemented on top of Steamworks.NET.
Steamworks.NET currently fully supports Windows, OSX, and Linux in both 32 and 64bit varieties. Currently building against Steamworks SDK 1.50.
Check out these sample projects to get started: