Personal Assessment
This involves taking note of & unveiling what you possess within you that can be packaged a solution to a problem(s) in a book.
To do this, assess your:
Knowledge
Experience/Story
Expertise
Profession
Market Assessment
Alternatively, big & profitable book ideas can be found through research.
This can be done on:
Amazon.com: find best selling books/topics here
Answerthepublic.com: find trending/current topics that you can write a book on here.
Facebook groups: Pay keen attention to observe the kind of posts/topics that gets the most engagements here.