Not having read Rojas' book, I think the best text I've ever read on the subject is Michael Nielsens (in progress) "Neural Networks and Deep Learning." [1] Chapter 2 covers back propagation.

It has a similarly approachable yet formal style, and I have recommended it to people with no ML experience in the past who have found it very intuitive.

[1] http://neuralnetworksanddeeplearning.com