I thought I would mention the EECS course notes from the MIT website: http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/index.htm
Of particular note, EECS 6.002 has all the course videos as well ! http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-002Circuits-and-ElectronicsFall2000/CourseHome/index.htm
That's 25 video lectures (about 50 minutes each) with all the course notes in PDF to follow along. The material is extremely relevant to whoever is interested in Arduino !
Having watched a number of the videos, I would highly recommended them to beginners in electronics. They would probably be a nice refresher course for experts too.