It's not clear to me exactly what such a proposed book is intended to cover, the extent of its coverage or the level of the audience it is aimed at.
One could argue it should cover 'everything' and through all skill levels, but by implication that means it fails to target any group which wants information pitched at specific levels without the clutter of the rest. And, as more is added, as its size increases and cost rises, more will be deemed irrelevant for particular skill groups, and it becomes less attractive all round.
On the other hand, any book targeted at a specific level is implicitly not targeted at other skill levels and is unattractive to them.
One thing we tend to have relating to PICAXE is an unusual situation where novices are often trying to do something at an advanced level. A boundary crossover which most books do not attempt to cross, and for arguably good reason.
It's also important to consider what an audience is seeking to get from such a book; the knowledge of 'how to' which leads them towards making an informed decision, or simply a 'buy this, that will do the job, connect it like this', or somewhere in between.
Also, people tend to be interested in a particular subject which they may well want described in detail from first principles through to advanced levels and have little interest in other topics which, while the book is in part ideal for them, may make it unattractive overall because of the cost or bulk of including the rest which they are not interested in.
Finally, you need to find authors who are capable of addressing every skill level covered and are knowledgeable in their particular subjects and skilled enough to do so, and to find authors who can cover a wide range of disparate subject matter, with the cohesion a single book needs.
Before commencing any book it's necessary to decide what that book is, what it covers, to what depth, and who it is targeted at. Is it a 'one size fits all' book or is it targeted at groups with particular skills ? Only then can one assess whether it is viable, commercially viable, and whether it will ultimately be useful or not.
