GUI programming

CS student here. Almost everything we've learned so far has been data structures and theoretical algorithms stuff. I want to actually make something with a user interface instead of a terminal.
What books would give a nice introduction to GUI programming?

>>49

Dot product

require "matrix"
def dot_product vec1, vec2, angle
  raise TypeError unless vec1.is_a? Vector and vec2.is_a? Vector and angle.is_a? Numeric
  vec1.magnitude * vec2.magnitude * (Math::cos angle)
end