Yes, as you stated, there are various design documents floating around. I believe that Al Lowe has some on his site. If you want to get a feel for working with the Sierra interpreters, there are some tools that let you create games using the original AGI and SCI interpreters. AGI Studio lets you make original AGI games and SCI Studio and SCI Companion let you create SCI games. SCI probably has more documentation.
http://sciprogramming.com/
http://sierrahelp.com/SCI/Wiki/
http://www.agidev.com/
http://www.agifans.com/
http://nailhead.org/agistudio/