This website I can say is probably the best website I have come across so far. It talks about all of the aspects and elements of math and the geometry in perspective drawing. It also has easy to understand descriptions.

there are many ways you can look at a drawing such as upward, downward, and eye level. for an example, you are drawing cubes, and looking at it from an upward perspective, the vanishing points will go downwards, making the illusion of looking at something higher than you. If you remember from my other posts what vanishing points (where the lines disappear, because it has gone too far away to see) and vertices (where two lines meet) are, this will be easier to understand. 

The left drawing shows looking downwards, and the right one is showing how the lines and cubes work when you are looking up at something.

 http://www.dummies.com/art-center/performing-arts/drawing/drawing-geometric-perspective/