The best resource on Niche Selection & Testing Criteria.

Hi guys,

First time poster here. Last summer I decided to take the leap and finally started my own online side-business. Even though it was a part time business, I took it with full time seriousness.

Well, after 6+ months of trying to make it work, I now fully resigned that this business was a failure.
I feel very good about the outcome because I learned a ton, and I didn't lose my shirt in the process.
It would not have been possible to learn the lessons that I learned without trial and error and hands-on experience.

I committed the classic mistake of building a product that I wanted to build, rather than focusing on existing problems and letting them dictate the product.

Anyway enough about that.

My most important takeaway from this entire experience for me was that the niche you select dictates the story of your business for months or years to come.

Poorly selected niche = no chance.

So I became an obsessive learner of what it takes to find the right problems to solve. As this is the most critical phase of a new business, I want to spend 2-3 months in niche research, preparation, and testing.

I come to you with the following question.

What are the most authoritative resources on niche selection and testing?

I have read and worked through several trainings from esteemed authors who themselves are accomplished entrepreneurs and practitioners of what they preach.

Will not name them because I also want to cross-check them on quality of the guidance they put out, but would love to hear your recommendations.

Sincerely,
Andrey