@cbruguera I've been traveling (8 conferences in 6 weeks) so I'm just now catching up with this thread. It poses some very interesting questions.
First, just to clarify, my definition of "connected thing" is any "thing" that needs to be referenced in a digital network. The "thing" itself does not need to be directly connected to the network. Your example of an identity document like a birth certificate falls into the grey area because the physical copy lives off the network but a scanned version would live on the network as a file.
But if we take your example of a book, say a hierloom family Bible that has been in your family for generations and needs to be referenced in a parent's will but has no connection online. That family Bible could definitely be given a dependent DID, with one of the members of the family as the guardian. The guardian could then pass on guardianship to other members of the family as the family Bible passes down through the generations.
In essence, that dependent DID becomes the digital presence of the family Bible (I've started seeing the term "digital twin" or "digital shadow"). The Bible is never capable of being independent, but as a dependent DID, it can live on for as long as it has a guardian.
That is my final point, which is that the DIDs and agent endpoints for "things" of any kind will always need a guardian, and therefore will live in the agency of the guardian. They can move between guardians over time, but the will never break their dependence on a guardian.