Yeah, I think I've read about a dozen of his books. But there's also such thing as "halo effect" (I think there's a book on this, too). Take the "i" letter for all Apple products now. It works the same way.
The halo effect makes it so all the children products benefit from the main product's influence, but it can also work the other way around, so they have to be very careful not to brand a crappy product like that, because then it will end up affecting all the others in a negative way.
It's also very important that they keep re-inventing themselves at least every 10 years (like Google is doing now). Because otherwise they risk becoming "the product my dad used", or uncool like that. Who thinks Yahoo is still cool now? 13-14 years ago they were "the Internet".
It essentially claims that a brand should not try to be all things to all people. Different brands, however, can. For a good discussion, see: http://www.codemyownroad.com/circumventing-the-law-of-line-e...
Edit: I see the book is written by the same author. Probably no coincidence.