How about real world programming FreeSWITCH.
Programming FreeSWITCH from basics to professional.
Most of the programming books I've read have a common program per chapter. Each chapter builds upon each other. And at the end you have a working application.
The theme could be building a soft switch. (part 2 could be an IVR)
Items to include:
Directory
Gateways- add gateway via code
Dial plan- lcr, limit, etc
Configuration
CDRs (putting it together) maybe the pop code as an alternative
Show how to create a module/ app
Show the examples in the most popular language
C, JavaScript and C# and have a download section for the examples for each chapter.
Last few chapters could be how to use a ram drive or connecting to a database.
Performance tuning tips and expectations of performance.
Maybe include the most common questions asked on the forum. And a blurbed about load balancers like opensip.
Just my two cents.