No one really knows an awful lot about Cascade - if you search for it on here you may find some pointers, but nothing at all definite.
The Leslie Piña books aren't terribly good for Holmegaard - they are, for the most part, quite vague on the subject, and there are a few big errors in there too.
Although it has its fair share of much smaller errors, the best book in English to cover Holmegaard is McConnell.