Look for a book called Monographs by Ben Cardall. He's spent his career learning the skills of observation and deduction so that he can be a real life Sherlock Holmes I've seen him do live-streamed demonstrations and it's really cool.
If you want to develop the genuine skills then you're not going to want to bother with most mentalism texts, and definitely no magic books. Mentalism as a field of entertainment is still based around deception for the most part.
You're going to want to look into books like "How To Read Every Body" and "What Every Body Is Saying". I honestly don't have much to recommend along these lines because like yourself I've already developed that skill independently as a kid. The FACS system is available for civilians as well - that's the Facial Action Coding System that Paul Ekman developed - it teaches you to read microexpressions. If you've ever seen the TV Show "Lie To Me", the main character in that show is based on Ekman and they feature his work frequently.
I will still recommend Psychophysiological Thought Reading by Banachek as CMR is a real skill.
And I will also still recommend the theory books I mentioned above - Maximum Entertainment and Scripting Magic - for the simple reason that theatrical skill is vital to a performance. More so than a bank of tricks.
And I still think memory stunts are cool so my recommendation for The Memory Arts stands.
Depending on which sources you studied for hypnosis you probably already have the bones of training for reading people - pacing and leading is based around the same skill set - so it may just be a matter of honing the skill and taking some chances with guesses for a while.
Yeah. That is, unfortunately, a thing. Learning stagecraft helps you quickly establish yourself as The Performer, but unless you get fairly famous this is probably something you'll have to deal with indefinitely.