What do you want to learn?
If you want to see a bunch of sample applications/circuits, the "famous" AN 31
is a great place to start.
If you want to make filters, there's the famous Active Filter Cookbook
, but that's an old pre-Intenet book and I assume that you can now find the formulas (to calculate the resistor & capactor values) online.
If you just want to learn the basics of "how they work", I don't know of a handy reference... I'm sure I've got a textbook on my bookshelf, but that should be available online too...