Filters—A Tutorial Overview", IEEE Trans. Microwave Theory and Techniques, v 51(4) April 2003 p.1368.
The discussion is on coupling in coaxial cavity filters, the principles apply equally to lumped-element resonators. Coupling is either magnetic or electric or a combination of the two. A well-behaved technique to create transmission zeros in band-pass filters is to introduce coupling between non-adjacent resonators.