XML is an widely adopted, open, non-proprietary extensible specification for text markup.
Its elementary strength (for text) lies in giving self-awareness (jikaku) to elements of the text. This is done by the distinct separation of descriptive and presentational markup.
Books in XML have a richness (from economic and usability perspectives) unparallelled in other authoring formats. The XML-based book yields numerous possibilities for enhancing the reading experience for everyone.
The new Talking Book is not necessarily audio-centric, but it is definitely XML-centric.
The new Talking Book emphasizes and recieves its strength from the jikaku of the text behind the talk.
The new Talking Book represents a movement towards format normalization, in that it is not based on a specialized format (four track tape, moon, braille) but rather on a format living at the center of concurrent mainstream technology. This has a (hopefully increasing) impact on the actuality requirement.
The Z39.86 committee is bringing Daisy 3 forward: XML grammar and media type extensions are two of the main focii over the next years (2005 and forward).