Source: colorful introduction to Linear Algebra (may 2014) Cosine Similarity – understanding it as volume of parallelepiped 
Another good source for understanding dot product : http://betterexplained.com/articles/vector-calculus-understanding-the-dot-product/ 
other resources:
Significance &Application of Cross product and Dot product. http://visualizingmathsandphysics.blogspot.cz/2013/06/vectors-significance-of-cross-product.html
Independence of Perpendicular components of the motion http://www.physicsclassroom.com/Class/vectors/u3l1g.cfm
Math Insight: http://mathinsight.org/dot_product
“projection” of vector b on a equals cosine of angle between them times the length of b or vice versa projection of a on b:
Dot product Identities, some pictures taken from http://gamemath.com/
