Show chapters in both book and part TOCs

I have a book with parts, chapters, and sections. I'm trying to show a TOC at the book level with parts and chapters, and a TOC at the part level with just chapters.

It seemed from initial inspection that I should be able to set toc.section.depth = 0 to include everything exception section titles. But this doesn't do what I want: at the book level, this just shows the parts in the TOC. At the part level, it shows chapters (this is correct).

If I set toc.section.depth = 1, it goes too far: at the book level, it shows parts, chapters, and first-level sections, and at the part level, it shows chapters and first-level sections.

So what am I doing wrong here? With a depth of 0, the book TOC shows too little, and with a depth of 1, it shows too much!

Thanks,
- Joel