Need or niche?
It is normal that most people know only a limited proportion of the plant species in any particular area, particularly in the tropics, especially if they are visitors. There is usually some useful or interesting information available about these plants somewhere in the world. It seems there is some need for field guides everywhere.
Maybe you are an expert on a specific group of plants. You may be inclined to create a field guide because you have something to tell the world, but you should rather work out how many people in the world might use - or even understand - the field guide you have in mind. Although your feeling of inspired enthusiasm will be useful for success, be wary of it blinding you to the real needs or interests of potential users.
The real niche for most field guides – the economic or market viability - is likely to be more limiting than any perceived need. When the cost and time of field guide production is added up, your planning phase will probably define a guide which is defined more on a ‘strictly need-to-know’ than on a ‘let me tell you this’ basis, even if you manage to show a funder of the merits of your guide and persuade them to subsidise its production.
Of course, marketing or education can generate or increase the demand, but for products like field guides, a plan led by existing demand is a safer proposition. Any increase in the demand that is later created by careful marketing or education will be a bonus.
On the other hand, you may not expect all, or even any, potential users or sponsors to be fully aware of the benefits to be gained by them from your planned field guide; you might then be able to factor in the increase in demand or budget expected from any linked promotion. It would be useful to see how much more positive a sample of potential users becomes if you take more care to explain to them how the guide will work for their benefit.
Purely commercial field guides tend to be more style than substance, or designed for sale in countries where people have a large disposable income. Brazilian, South African or Malaysian field guides obviously have more chance of being commercially viable and accurate than field guides for Mozambique, Cambodia or Mali, where potential users are in general poorer. On the other hand, there may be more chance of persuading NGOs or development or conservation agencies to subsidise field guides for poorer countries