This extensive tutorial provides a thorough foundation for the understanding of piezoelectric/crystal resonators properties and behaviors. The table of contents provides insight into just how comprehensive this tutorial is:

1. Applications and Requirements
2. Quartz Crystal Oscillators
3. Quartz Crystal Resonators
4. Oscillator Stability
5. Quartz Material Properties
6. Atomic Frequency Standards
7. Oscillator Comparison and Specification
8. Time and Timekeeping
9. Related Devices and Applications

Section 9 on related devices provides some understanding of SAW and BAW filters as well. 

This particular archive, on the web site for the IEEE Ultrasonics, Ferroelectrics and Frequency Control society has the complete set of materials. This  includes brief video animations of some of the crystal vibration modes which are omitted from other sites hosting this particular resource.