I know how you feel. I've searched for any general information guides on HL programming, but the only things I can find are useless copy/paste "tutorials". If anybody knows any HL programming guides, I'd love to know about them! There's nothing I can find even on the basic stuff, like client/server messaging, adding new entities, locating objects, AI scheduling, or any general architecture information.
It's all really specific stuff like: "make your gun glow in the dark: just paste this code in this file!". It's annoying how inefficient this stuff is - instead of someone writing some useful documentation, everyone's trying separately to read Valve's terrible code and trying to figure out how it works.
I haven't done a lot of HL programming, but for adding new weapons, the only thing I can suggest is: copy the weapon that's "closest" to the one you want to make, and then start modifying the code from there. But that's not very useful advice