Hi @mbelair – the two level TOC should work with any theme that supports it (all but a very small number, I think). The idea would be that you simply designate top level headings as H1s in the chapter, and they’ll have links automatically created and displayed in the book’s ToC, much like the guide itself, if you look closely.
You can make this collapsible/expandible, too – there’s a web setting that lets you collapse/expand the top level sections. I’m afraid there’s no way at present to automatically generate additional levels within the Pressbooks interface at present. You’ll notice that parts themselves can contain content, however, so maybe that will stimulate some additional structure ideas?