Reading @clement's post on writing a book brought to mind a couple of ideas I've found useful when it comes to writing, which I thought I'd share:
1) Don't Aim To Be Comprehensive
Don't try to cover an entire topic. It's too overwhelming and it becomes more of a "landscape" book ("this is the topic and all its areas") vs a "pathway" book ("this is what you need to do to accomplish X"). I like to cover the landscape a bit to point out the pathway.
What specific problems/opportunities can you help address?
What can readers do with the knowledge you've equipped them with?
What transformation will they experience in their lives (or businesses)?
Brainstorm/research problems people complain about or benefits they desire, and aim to tackle 1-5 of these. You can go for more if the book will do them justice without leaving readers overwhelmed or scattered.
2) Develop An "Umbrella" Brand
The tagline of my personal blog is: "Not a pigeon". That's because I hate to be pigeonholed. I know I have multiple interests and I don't want to define my personal brand by a single interest.
Coming to terms with this has allowed me to write about multiple topics without the fear of diluting my brand. My brand IS in having multiple interests. At least that's how I want it to be. :D
Besides, the vast majority of people have multiple interests, and they usually have a few shared interests with others. Doing one thing then choosing to pursue something else doesn't necessarily have to be a bad thing. You can make small contributions to multiple fields.
I hope these ideas are useful to those who are struggling to define the scope of a book project or how they wish to be known.
And good luck to all writers!