RED (robust and efficient DSP) is a collection of algorithms that should help engineers to implement basic signal processign tasks in - as the name states - a robust and efficient way. This includes mainly numerical aspects. An overview about the RED collection can be found here.
Here you can find the chapter on the design of so-called crossover filter as a single pdf file. If you would like to download all chapters please go to the RED overview page and get the pdf document that contain all chapters from there.