Many people want to write an e-book but get stuck on the subject. The original article suggested starting with your interests and knowledge. That helps, but a topic becomes stronger when it also answers a question your intended reader has.
From interest to question
List what people often ask you to explain. Which answer do you give repeatedly? Where do beginners make the same mistake? An e-book about “marketing” is broad. A guide to writing a first product page has a clearer purpose. The subject must be large enough to explain something and small enough to finish.
Then decide whom you are writing for. A beginner needs a different explanation from someone who has run an online shop for years. Note what readers should know beforehand and what they should be able to do afterwards. That stops you putting everything you know into one book.
Test the outline before writing
Draft a contents page with a few chapters. Can you name a specific question and example for each? If not, the topic may be vague or need more research. Show the outline to a few people in the intended audience and ask which parts they would read and which question is missing.
A good topic need not be entirely new. It should address a recognisable problem from a useful angle. Start with a clear promise rather than an impressive title. Writing becomes easier when you know whom each chapter serves and which decision or action it helps them take.