Best way to learn how to maximize deductions

I have a single member LLC. I know it's a complex topic but I would like to know how to better maximize  my deductions and save on taxes.

For example:

- how to know if a business expense is a write off or not?

-is it better to lower my taxable income through investing the money I made that year into my own business or contributing in a retirement account . How do I calculate how much should I invest/contribute at the end of the year to save on taxes?

 

Other than general advice I would like to know if there is any book, video, course, or any kind of  resource that could help me to learn more about it.
Thank you