Konstantin describes a node hierarchy system powered by nested sets. Thats far more powerful than Ber's relations as described in those links. Nested sets allow you to express complex hierarchies with excellent performance. Thats not true with flat relations table.
Many of us in Amsterdam discussed nested sets as a promising solution to our widespread neet for hierarchy. I encourage Konstantin and others to pursue this solution. Book.module is also excellent test case IMO.
I'm compiling a writeup on a variation on the nested set algorithm, which should help with frequently changed nested sets - to avoid frequent re-indexing. I think Aliie, has a prototype of a hirarchy module in her sandbox as well (vauxia?) I think it would merit thinking through an approach, where we use "specialised" indexes for different ordering types like linear, tree, graph, etc... Cheers, Vlado