The Bogleheads Guide to Investing was the thing that helped me the most. My family got me prepared for day to day finance, but nobody explained retirement savings to me.
This book broke everything down into easy to understand terms, and made me feel comfortable investing wisely.
The book used to be free online, but I don’t see it anymore. You could search up a PDF, or they have a really good wiki that covers most of it, but more briefly. Here is the Getting Started page.