Why don't you use the documentation at openacs.org/doc and improve it?
To improve the documentation you would first have to know docbook fairly well. Then there are the additional steps of compiling the information and putting it up somewhere. Docbook is a great method for actually writing the book in the end, but for the moment, where all we need is an easy way to collaborate and collect documentation from various places, docbook is overkill.
Should the documentation become obsolete?
No definitely not, it is after all one of the wealth of information. Sadly, it has not been update for quite some time and things have evolved since then. Our goal is primarily to provide up to date information in a structured way that makes it easier for new developers to pick up OpenACS (and the reference documentation is not written in the best style to achieve this, in my opinion).
How should the book differ?
Using real world examples to tackle common development problems as a guideline we should introduce the user to OpenACS. Much like the notes tutorial, but with more information on each of the various parts and deep links into forum postings, the OpenACS documentation or other places where information is kept.
