As you say, there's always good points and bad points with any organising method
The main reason I went for this approach was that when I was starting out, I found it very confusing to have to keep flipping backwards and forwards through
gardening books to work out what I
should be doing when. Either in terms of "what can I plant or do right now?" or "I want potatoes; when do I need to put them in?". I thought having it laid out by the month would make it easier for beginners, and this is pretty much aimed at beginners.
But yes, temperature zones and all the rest do make a difference. I'm in the southern UK and based this around my own balcony, but I did include some zone/hardiness info in the introduction and talked a bit about altering suggestions for different locations. And I tried to make similar comments throughout the text. With container gardening you're often dealing with microclimates anyway which can affect those decisions -- I was growing at the time on a south-facing balcony on the 1st floor of a
concrete building, which meant that I had different (warmer!) conditions from even someone with a north-facing ground-floor garden on the next block along (or, indeed, the south-facing tiny garden I now have half a mile north of that flat!). And I think containers can have more in common across different locations than in-ground growing would in those same locations. So I hope I've included
enough information about adapting for
local conditions to make it useful in other locations too.
Sales-wise: yep, possibly, but again, you have to make decisions, and being very wide-ranging can affect sales too. I discussed it with the publishers and we agreed that the benefits outweighed the disadvantages, especially as above with discussions of how to make things work in your specific location.