The real missing thing in Wikibooks is a way to define the structure of the book, in a way understood by the software. Currently the only notion of structure for MediaWiki is the parent-page/subpage relation, but this is clearly insufficient. All the book contents, the ordering of chapters (next, previous chapter), even the chapter hierachy if present (up, down chapter), should be provided using wiki syntax and then profited by the software in several ways. This is something I think comes implicit in Magnus' proposal. As I see it, the important concept of his idea is that of structure and navegability rather than the viewing mode. The later is something obvious once you have the first point implemented.
There is an extension already developed and available in [1] for implementing this concept. Once we had the book structure defined, MediaWiki and external tools, could make use of it for several applications: better printable version, automated all-content pages, automated TOC for the whole book, etc. I definitely think this is something missing in MediaWiki for clearly fitting the Wikibooks goal.
Regards,
ManuelGR