The two-part blog post on clamping:

https://sinmantyx.wordpress.com/2015/03/18/perfect-clamp-1/
https://sinmantyx.wordpress.com/2015/03/18/perfect-clamp-2/

Sampling and Light Paths by Blender HD:

https://www.youtube.com/watch?v=NhOdnOow6DY

Disney's Practical Guide to Path Tracing:

https://www.youtube.com/watch?v=frLwRLS_ZR0