Where do I find the API reference for the Half-Life SDK?
There isn’t one (yet). All of the tutorials and articles available on the Internet came from people who were willing to sit down with the SDK source code and determine how things worked for themselves. When they learned how to change something or create something new, they took the time to write a tutorial so that the rest of us could benefit from their effort. Just like I took the time to sit down and compile this list of common questions about creating a MOD for Half-Life. Maybe one day there will be a complete list of all of the classes, functions and variables found in the Half-Life SDK with detailed descriptions on each of these, but until this happens, the best way for you to understand what can be done is to examine the source code yourself to try to understand how things work.