Need: something people would be much better off if they had (or
even, will struggle to survive if they don't have). Example: fresh
vegetables.
Demand: something that people are actively looking for, and will
pay money to get. Example: McDonald's.
If you want to write a book that sells (or create any other successful
product), figuring out if there's a demand is more important than
figuring out if there's a need.
Lots of security and software engineering books deliver on a need. But
is there a demand? Do people really
want to buy books on those
things?
Valuable, ethical products and books deliver on both. How can you
present your useful, important book in such a way that people will
want
it?
