Sadly I've found that most sources prefer to be academically rigorous over quickly comprehensible. Of course it's good that people have done the academic work, but it can be tragically comical how obtuse an academic paper can make a simple concept.

However, one of the most recommended books on the subject is available online, so you might want to check that out.

Computer Vision: Algorithms and Applications by Richard Szeliski

http://szeliski.org/Book/