Value of published (non-english) books for applying in a PhD program
I don't think the language matters. It is a substantial achievement, I think. If it is in your field, which I assume, it will have value. But, depending on where you are applying, you will need other things as well. If your contribution was "only" providing exercises, then it might be a bit less, but for content contributions it would be good. 

But no one thing is determinative of your suitability for admissions.