Because my side projects did not earn me any money yet, and because I don’t want to spend too much money on them, I did not hire professional designers for them. This might change in the future, but until now I did (almost) all of the design myself. I had to learn a lot to be able to do that, and I think I am getting better, but I am not a designer - and I’ll probably never be one.
There are two books from which I really learned a lot: The non-designer’s design book by Robin Williams and Visual language for designers by Connie Malamed.
The non-designer’s design book contains lots of useful tips that you can immediately use in your daily work. It explains four basic principles of design in a way that is easy to understand and remember. The book also explains the basics about type and typefaces - things everybody should know.
“Visual language for designers” is more about the big picture. It explains how we process visual informations and how designers can use this knowledge to create effective designs. The book contains lots of examples. It is sometimes a little hard to read and does not contain many practical tips or workflows, but it is definitely interesting and I am sure that I can use the things I learned from this book in my future designs.