@Daanii I have two arduino Duemilanoves and love them! I just feel that if I had a stronger understanding of electronics I could go to new heights with them.
Just my own two cents, but I think you would learn more experimenting with your Arduinos than by hitting the textbooks.
For example, have you learned to read datasheets and tried out a variety of devices? For a project I'm working on, I had to work with a digital potentiometer or digital-analog converter. Experimenting with them, I learned a lot about SPI serial communication, resistance, and voltage. I also learned about how temperature affects resistance, and how to filter the 5 Volt power to limit noise.
If you want to learn more about things like motors, you could build a robot. The digital side would still be interesting. But you can also learn about power transistors, H-bridges, and batteries. All important, useful stuff.
But maybe you've done all of that already.
Problem is, compared to practical experimenting, circuit theory is boring and largely useless. Kirchhoff's laws. Thevinin equivalents. Nodal analysis. Your mind will boggle. You have to learn it to go on to more advanced electronics, but it's not practical stuff. It's not going to help you do anything new with your Arduinos. Not even close.
That said, one thing you may want to do is look at the MIT OpenCourseWare lectures, and do the homework. ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/ One problem is that there are no answers to the homework. Plus you need the textbook (which I did not have) to get the most out of it. But the videos offer a good, free survey of basic electronics and circuits.
One textbook I came across and thought was pretty good was Introductory Circuits, by Robert Spence. My advice with any textbook is to do the homework. Don't try to read the chapters and learn from that. Flip to the problems, start working on them, and refer to the text only as needed to do the problems.
Whatever you do, good luck!