This is an environment that creates a way to use or even create Windows .NET framework applications on MacOS X and Linux. Some of the eBook utilities are written using the Windows .NET framework, C# for example.
Mono is a software platform designed to allow developers to easily create cross platform applications. Sponsored by Xamarin, Mono is an open source implementation of Microsoft's .NET Framework based on the ECMA standards for C# and the Common Language Runtime. A growing family of solutions and an active and enthusiastic contributing community is helping position Mono to become the leading choice for development of Linux applications.
Mono itself is an open source, cross-platform, implementation of C# and the CLR, Common Language Runtime, that is binary compatible with Microsoft.NET
MonoTouch for iOS allows you to build apps for iPhone and iPad using C#, MonoDevelop, and the Mono Framework
Mono for Android allows you to build apps for Android devices using C#, Visual Studio or MonoDevelop, and the Mono Framework