I like this site for analog stuff. Covers stuff I would look up in my text books from Sophomore year that Covered Op Amps. http://www.electronics-tutorials.ws/opamp/opamp_1.html
The other Sophomore year course that comes in handy for this stuff was COLD, Computer Organized Logic & Design.
Would address the relatively easy digital stuff arduino does - reading switches, driving LEDs, driving motors, interfacing with shift registers, etc.
What comes in the handiest tho is just reading the data sheets. Don't rely on libraries - read the data sheet, understand the signals & commands needed to set up internal registers, etc.
Application notes for chips are useful too.