So my younger son apparently thinks he has too much cash on hand and wants to talk to us about investment advice. He's in the Navy and I believe he squirrels away the maximum they allow, but not the one where you have to spend 20 years to get the full pension. We'll ask again when we talk to him. Generally my advice has been maximize whatever you get from your work place. Put it in a Roth IRA. Index funds are the way to go. Live within your means. Pay your credit card every month.
When I was his age or a little older, I really liked Jane Bryant Quinn's Making the Most of your Money. Would love to give him something similar.
I have no idea if he should be worrying about Covid-19's eventual effect on the stock market. I don't understand why it hasn't crashed yet!