A friend of mine always used to ask themselves 3 deceptively simple questions when evaluating a book proposal:
1) what’s it about?
2) who is it written for? And,
3) what will they get out of it?
Often authors are clear on the ‘what’, but not so much the ‘who’. I’ve frequently read statements like this on proposals: ‘This book will appeal to men/women of all ages and could be read by anyone…’ Or, ‘Is aimed at adults and young people, appealing to both audiences…’
What the author is trying to do is broaden the possible audience as much as possible, in the hope that the publisher will consider it a no-brainer to take on. Actually what this says is: I have no idea who my target audience is…
My advice is that it is better to identify a clear target audience, even if you consider it to be quite narrow. If the publisher decides to take your book on, it will make the marketing of the project so much easier.
Also, decide on your target audience before you begin writing, so that throughout the development of the text you write with that type of person or group in mind.