I know this might seem funny, but try Wikipedia if you haven't already. It's really comprehensive, and even senior level Electronic Engineers (edit - students) use it heavily.
It doesn't always work out this way.. but because it's meant to be general, the articles usually have a simplified, down to earth, easiest possible route to understand. Every time something seems unclear to me at one place, Wikipedia seems to fix it up for me.
Sometimes it even has a general article and a technical article for the same thing. Operational amplifier, for example, has a history and development article. It links at the bottom to another article, called "Op-amp applications" which gives all of the equations and configurations for different op-amp tasks.