For strategy and vision, I can highly recommend starting with these two books - "Creative Strategy and the Business of Design" by Douglas Davis and "All Of The Other Marketing Books Are Crap" by Denise Kohnke.

With delegation skills it's a bit more tricky, you can certainly find many books on that topic int the business world, plenty from Harward Business Review for example. But, I think the practical side needs to be explored just as much as the theoretical.

To practice, come up with a simple design project, let's say a landing page. Work on the creative brief and a project brief (from the two books above, you'll learn that these two briefs are very different, and writing each takes time and effort). And then, find a freelancer to get it done. For the sake of experiment, go with Upwork or Fiver, so you are not spending too much of your own $ to learn on your own mistakes :)

You can also add more complexity where you are working separately with a copywriter, designer, and developer. This experience will most definitely elevate your project management and delegation skills.

Hope that helps!