@Talyn: I calculated the profitability of the categories and not the niches. Have my numbers not here but you can calculate it by yourself. I also did it so everyone can do ist.
@behemothseo
1) When I start in May this year I was really really hungry to make money. I already had a stable passive income but when I have seen the numbers of other successful publishers and thought about the possibilities I was impressed. I have written a series of 4 books within my first month (about 100 pages each book) and published a more expensive 4 in 1-collection. During my second month I published 2 or 3 books in other niches and so on. I also create paper back versions of my books within the next month. Today I am writing on a new series of 3 books (+ a 3 in 1 collection) in my most successful niche. This series will be amazing and you cannot differ this books from official/professional publisher books. I also want to create paper backs with a nice layout. I think I will need about 2 or 3 month until I am ready to publish. But I already know that this niche works and I think I can double my income with this series. With these HQ books I have also not be scared of bad reviews and so I make sure that I will make money for long term. So my intension for the future is to hire cheap ghostwriters ($100/book) to make fast money with fiction books but also to test new non fiction niches. And If I see that a niche is working I want to write some high quality books by myself. I don't enjoy the writing part. It's pure stress for me. So for now my work is more delegating my writers, creating the covers (I love this part), formating the books and analyse the marked. I also spend a lot of time with reading books about self development and so on.
2) I'm not a native English speaker. I don't understand your question. Q/A? Question and Answers? I dont know what/why should I ask? Do you mean where I get my content from?
3) My first series has about 15-20 reviews/book. The average rating is 3,5 - 4 stars. But I have also published bad quality books where I got many 1 star reviews. But I have written these books in 2 or 3 days just to test other niches. But because of Kindle Unlimited I also make about $50 with such a bad book.
@vypersells
I agree with you. I think everything is already answered in this thread. But most of the people are lazy. They want to make money without of work. They don't want ...no, they are not able(!) to think a alone. They want a step by step guide with a guarantee to have success. And there are tons of such guides but at the end all these guides are senseless if you have a weak personality and/or If you are not able to work/think alone. That's the big difference between an unsuccessful worker and a successful business man - the personality and the effort you are willing to cotribute. Sounds evil but that's the truth.
In my whole career in the IM business (also before selfpublishing) I teached about 10 of my best friends how to make money. I have told them everything. I have supported them 24/7 and answered every question. I have even helped them actively and build them up a passive income. The reason: I wanted that the people around me who I love have the same success like me. I wanted that they are also financial free and con do whatever they want. So they really really had the best requirements to be successful. I did everything. The sad truth is: 9 of 10 people fail. They really wanted to make money but they were not able to work and think alone. If I asked them "Did you checked out this amazing forum post or that YouTube video" - No, no, no, never. As long as I did the work and answered them everything it worked for them. But when I have needed the time for my own business and could not report them 1 on 1 what they have to do next ...they failed. Some of my old friend studied but they have no job today. Some of them have a family and doin a job that they hate - all of them are struggling in another way. That makes me sad but I cannot help them and it's not my fault that they are broke today.
Even my dad losts his job after 25 years in his company and it hurts and breaks my heart when I see what jobs he have to take now. He had about 10 different jobs within the last two years and sometimes he just worked for less than $5/hour. This whole system is so ugly. But I am not able to help them. I connot teach them how to make money because he has no business mindset. He's not that pesonality to found a business and he has also no big visions. He is a typical worker who thought that a 9 to 5 job is the safest way (but time has changed). So the only chance I have is to be succesful to share my money with my parents. My biggest dream is not to become a millionaire and live in a big mention or shit like that. My biggest dream is to put my parents $500k in her bag sometime and say "Do what you want. It doesnt hurt me". Thats a mighty mighty intension and the reason why I stand up every day and work hard. And that's also the reason why I will reach my goal ...not tomorrow but in less than 3 years I swear to you.
So belive it or not but I think that 90% of the people will fail and never reach a stable passive income. Even the people in this forum will fail. They are looking for instructions with dollars in her eyes but success only depends a strong personality. If you are a strong personality you will have success in mostly every business even outside of the Internet because every business is mostly the same (find a niche, create products, market and sell your products).
Before someone is screaming: I'm not god and I also have my faults (eg. actually I can not handle the taxes thing because I am sooo lazy right now to hire someone for this task. I am just looking forward to the release of Assassins Creed Unity and Christmas holidays with my family loool). But I think I can afford it.

So I just want to motivate other people with my posts here and maybe something sound a little bit harsh ...but I just want to open your eyes. Don't ask stupid questions (and yes, there are stupid question nevertheless what the school or anyone else tought you) and use the chance to ask something where you really need advices.