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...