What are you looking to do?
I don't think "Client Plugins" are actually a thing, CastingEssentials hooks into IServerPluginCallbacks (in engine/iserverplugin.h) from what I can tell; from there you can pretty much do whatever you need
I don't think anyone has ever fully documented how shit actually works after reverse-engineering; you can probably find snippets by googling random topics or class names.
I've dabbled a little; it's a lot of poking around once you get hooks working
You're honestly probably better off using CastingEssentials as a base
Depending if they can afford to spend time with you, Pazer and SIGSEGV are great sources of knowledge, the AlliedModders wiki and forums as well
I'm gonna say the following, not to discourage you, but as a warning for the sake of your mental health:
Working with the source engine takes an insane amount of patience, you're battling with C++ code that's almost 20 years old, most of this shit doesn't make sense, it's probably not worth your time