Arduino is a multipurpose, flexible platform, there is no ultimate manual with everything you need to know because circuits and code depend on the components you use. Luckily, there is a vibrant culture of sharing around Arduino so you can find plenty of information on the Internet that is not tailored for engineers and electronics geeks.
A good start for everything Arduino is the official site. Search the forums if you encounter problem with code, chances are that your problem has been solved already.
If you want to go step by step and learn to use all the most common components you should follow the Arduino Starter Manual. We should have all the material they use in the lab.
For everything else the best resource is Instructables and of course Google, look up Arduino + anything.
Datasheets are the main resource for electronics, every component has one, but they can be very technical and intimidating. Typically you just need a scheme that tells what pins do, where is the ground and the vcc etc.
In any case, if you have a component in your hands and you don’t know how it works don’t try to figure it out by trial and error (that kind of never works), instead look up the code that identifies it. The code should be printed on it somewhere.