The learning curve of your writing career, if you want to save time, should look like this:
The order matters. Learn to tell a story well, then learn to produce a body of work, then learn to sell. Master where you are before you move on.
Otherwise, you’ll produce a stack of badly crafted books, or spend a fortune on advertising such a small number of titles that you have no chance of breaking even.
If I were to choose sources for these three stages, they would look like:
What is my wish for you?
Write well. Write fast. Sell books.