I am by no means an expert or a financial advisor, so this is just how I do it;
Invest what you are comfortable with every month into a low-cost index fund.
It’s about time in the market, NOT timing the market.
Ideally, you want to have an investment horizon of 10 yrs, and the longer the better.
Resist the urge to tinker.
For books there is an entire thread dedicated to it here
All the best,
BTY