GraphicsBook.sln is a Visual Studio solution file containing several "projects" such as "Testbed2D", a library of components used for writing small 2D programs, and Subdiv, a small polygon-subdivision program written atop Testbed2D. Each project has its own README file describing it in greater detail. To get started, open GraphicsBook.sln in Visual Studio, and choose "Build" repeatedly until there are no build errors.