Marketing is a big one for me. Building a great product means nothing if you can’t reach your target audience. And I don’t mean learning how to used Facebook or Google to run ads, I mean the theory behind marketing. I would like to read more books similar to the 22 immutable laws of marketing by Al Ries and Jack Trout
You might actually learn more by running a Google ads/FB ad campaign for a side project. Put some money on the line and I can almost guarantee you'll be more committed to learning than if you were just reading a book.