Intro to design or where is a good place to start

I’m a self taught furniture craftsman and I am looking to expand my skills. I started my business offering a very simple bed design. As I am starting to grow I am often asked to create custom pieces. Since I have no formal training I search for pictures of what the client may want and draw up something in sketchup.

What I’m hoping this community can offer is where I can begin to learn the basics of design. My formal education is business and technology. I’m very analytical so the creating process can be challenging for me.

What are some good books and/or online courses that I can take to expand my design skills?

Thank you from an emerging craftsman