Learning complexity takes time...bottom line. Eventually you'll have no problem looking at the .h file and figuring out how the interfaces are designed and intended to be used, but it does take practice. It's impossible to explain it all in one sitting, it just is. The volume of material is large enough that it must be learned piecemeal. The only way to truly grasp it is through practice and repetition, failing and fixing. The good news is that the vast majority of programming involves repetition. When you do the same basic thing over and over again, once you've done it a couple of times you've got it down. After a few hundred times it becomes instinctual.
My suggestion is that you open the .h file and start reading it. When you get to something you don't understand, post a question and get someone to explain what's going on. I will do that for you if I can. Then you'll learn then next piece and eventually you won't need to ask anymore. Frankly, if you can understand and write working code without using a library, then it won't take much more effort at all to understand what is being done by a library-- which in the end is just a method of taking related chunks of your code and separating them out into different files to make things more organized and maybe reusable in some other project. The code isn't any different. I mean it doesn't follow any different rules just cuz it's in a library file. It's still C++ through and through, just not all stored in one main program file is all.
Something you have to resolve yourself to accepting is the fact that hardly anyone documents code for free. It isn't really worth it. It has to do with laziness, not conceit, by the way. Be wise and learn to read it yourself so you can do without the docs, otherwise you are in for an eternal struggle. Just like with anything that requires you learn how to do it. If you refuse to learn how and try to stumble along anyway, it makes doing it necessarily unreliable and confusing and hard to repeat. Don't believe me? Try running a bulldozer without learning what all the buttons and levers do.
What you are trying to do is, in fact, much easier than being able to understand the entire library. You just want to use the library functions and are unconcerned with the details of how they work. All you need to be capable of is looking at the .h file and identifying the functions defined in there and the parameters they use. Then try them out to experiment and find out what works and what doesn't. So initially, just look for function prototypes and you'll come up with a list of possible actions you can invoke. Functions always conform to this prototype structure:
returnValueDatatype functionName( parameter list );
so that's the pattern you want to look for in the .h file. Locate all of them and you'll have a good understanding of the interface available for you to use. Then just experiment if there is no doc. Post specific questions when you get stuck. Pick it up a little bit at a time.
You haven't really asked a question in your post. It is more of a rant really. And so you are not likely to get any sort of answer that solves your dilemma. We can commiserate with you cuz we went through the same thing, but we can't insert the understanding into your brain through some sort of osmosis process. I can tell you for sure there is absolutely no "trick" involved with reading libraries. It is no different than reading any other C++ code. Just a basic understanding of C++ is all that's necessary to do what you are wishing you could. If you ask a specific question I think you'll get a more concrete useful answer that actually furthers your understanding and capability to figure these things out yourself. I assume that is your goal.
Stated another way, don't ask how to assemble a jet aircraft. Instead start by asking how to assemble a wing or a landing gear or some smaller piece that can be reasonably explained in one short sitting. Find something specific you don't get and ask about that first. Trust me, the rest will fall into place as you go...and it will get easier and easier to understand when you progress this way. We can't help you step through it if you don't move in reasonably small steps. Walk before you run and all that sage advice. "How do I read a library?" is far too broad a question to be answered...especially in a forum environment, but even in a face-to-face situation. All you could hope to get by asking such a thing is an equally broad answer: "By decoding them line by line".
