I've similarly wanted to start learning some basics of reading financial statements. A friend of mine in a financial role recommended the book _The Basics of Understanding Financial Statements: Learn How to Read Financial Statements by Understanding the Balance Sheet, the Income Statement, and the Cash Flow Statement_. When asking how to "read and understand an S-1", I assume you mean the financial statement figured within, so I think that book may be beneficial to read, although I've not yet read it. If someone has further recommendations geared towards S-1's or other financial statement reading that would be great!