Our team has learned a lot while developing Insight3D and the 3D capabilities in STK. Designing a 3D engine that meets the precision and performance requirements for these types of applications is challenging, to say the least. It takes a carefully designed engine to do things like visualize immense amounts of imagery and terrain, and precisely render a massive world.
Deron and I realized that not much has been written about this type of 3D engine, at least not all in the same place, so we decided to start writing a book on the topic: Virtual Globe and Terrain Rendering. This isn't a book on Insight3D or STK specifically but rather a best practices guide for designing and implementing 3D engines for virtual globes, GIS, simulations, etc - all things we've learned by working on Insight3D and STK. If you're interested in this kind of stuff, check out the book's blog as we continue to work on the manuscript and example code.
I should mention that our book is not directly affiliated with AGI. Although, since AGI is such an awesome employer, they are very encouraging about our book project and allowed us to mention it on this blog.