To citizen,
The resonance theory is very interesting, there a different types of resonance and different types of tanks too. They allow having diverse resonant effects and getting very unusual characteristics. It can be used not only for filtering but for correction or equalization as well. I have one excellent book: Everitt W.L., Anner G.E. Communication Engineering. My book is the 3rd edition (1956) but the text was first published in 1932. There are chapters about resonance, RF networks, filters, impedance matching (even broadband), equalization and coupling. It may be hard to find today, but text is very good.
The coupling in RF filters is separate area and it is very interesting, but not easy. There are some hard to believe phenomenon, but they really exist and it is possible to see them. I know one wonderful book on this topic, but it is not in English. It was written by one of my teachers, he died long time ago. I use this book often when it is necessary to refresh something.
To LvW,
One of your post above remind me about a couple of very interesting things. Several years ago I invented the way how to make solid state amplifiers with resonant characteristics without any inductors. I checked it with simulations because I have no ability to make ICs in my garage and simulations showed good results. After it I developed new RF element what I called Bell-shaped tetrode. It is semiconductor device well suited to IC process and it has Bell-shaped characteristic. By changing bias on the first electrode it is easy to adjust the height of the bell. Steepness of the positive and negative slopes can be easily adjusted with biasing of another electrode (this is a tetrode). Simulation showed good results. Bell-shaped tetrode can oscillate with very small current consumption and has low loading on the resonant tank. With bias it is easy to tune the tank on desired frequency. The Q-factor is much higher than for varactors, so phase noise can be kept very low. I think there is a real possibility to build RF circuits even for GHz bands without inductors at all. This is my very old goal: how to get rid out of inductors. If it is interesting for you we may talk a bit more detailed through e-mail.