While reading this ... at first I was very much going to give the 'surface' answers (which I will still give later in thia post) but then, as Xalle and Pomona posted... I got to thinking, Yeah they are right, it is a HUGE subject. Complex to the N th degree and thus very hard to reduce to a few simple lines of text. But that complexity does not either detract from its beautiful simplisity, but neither does it help to explain it or answer your questions...
But... I'll try...
'What is it?
How does it work?
Where does the power come from?
How do you do it?
How do you learn how to do it?
What do you do magic for? e..g can you do it for monetary gain etc?
Is it dangerous?'
It is life. It is the sun rise and set, it is birds in flight, it is the lion at his kill, it is the mother with her child, it is your evening meal. It is the energy that makes this world / universe be HERE. But all that is all too big for a mere mortal to deal with... so it is also the little touches that you can make, the will that you can muster to change the little things without apparent physical interaction.
It is the application of imagination and will.
The power comes from both within and without. Within it is the strenght of me, my mind, my will, my life.
Without it is the strenght that is lent to me by everything from a green lawn or a moon scudding in the clouds, a thunderstorm or a Divinity that feels inclined to answer my call and offer thier help.
There are a million different ways to do it... but I tend to keep things simple. I prefure to be outside, I prefure to be alone, I tend to have thought deeply about what I want to achive for quite some while before hand... sometimes months, and this is also part of the magic/spell, but I don't tend to have a fixed ritual or plan per say. I rarly case a 'formal' circle, but I do prefure to have a 'space' in which to work. I normally introduce myself to the world at large (here I am, hear me please, suport my magic, I open my heart so that you see my full/true intent) as part of that 'space' creation. I get in the right mood, breath, be still with the thoughts of what I am doing, what I am going to achive. Sometimes I speak, sometimes I am just still witht eh thoughts/will/magic. It is like touching gossomer threads of light and just changing the pattern... When it is really working (for me) it is almost physical, I can 'feel' 'see' 'smell' 'touch' 'hear' 'taste' it. I find it works better if the changes are subtle, working with nature / natural law, but it is rule bending and there are always consiquenses.
It is not something to be lightly or friviously undertaken. So while I could use it to gain money and the like... I doubt I ever would. It would not be worth the effort/consiquenses. I formally work with magic, as in casting a spell VERY VERY rarely... as in I have been on this path for well over a decade and I know I have only got this deep into magic less than five times.
'Small stuff' (not that there really is small stuff...) like putting a wish for strenght/ health into a meal you are serving or blessing a patch of ground where you are planting, or (as ozz says) the wish as you blow out the candles. Well that is almost daily. Pehaps like a Christian saying grace before eating or a prayer before retiring to bed... normal, instinctive, part of life (which is why I say magic is part of life / is life).
There is danger involved in using magic... not prehaps as crude and obvious as a gun, but more subtle like the over expectation, arragance, the looseing of Self. Which are just as deadly as any gun in the wrong hands. When you work with something as potentially BIG as magic, as the Universe... there is a risk that it will slap you and if it does that is going to HURT. Just a matter of scale.
As to how I learned... well I am self taught, and I just followed my insitncts. I believe anyone can do magic, that humans are magical, but most of us shut off from it. Thinking that it is just wishful thinking or something for children and fairey stories, or do it without awareness, calling it Luck and so forth.
Hope that helps somewhat...
As for reading, well Harry Potter magic is very much fantasy, But a lot of Pagans have some kindred feeling with the magic as discribed by Granny Weatherwax in various Pratchett books... so prehaps you might enjoy those as an 'educational' read.
Be bright, be bold
Fillionous