It really depends how deep you want to get...
If you want overview with basic knowledge(to level of building iem)
You can use
@Furco book.
OK
Here is my classification
Level Basic
1.
@Furco compilation book
2. Playing with dampers
Level intermediate
1. Extreme knowledge of passive circuit in signal. Get any electrical and electronics book
2. Good and thorough understanding of Balanced Armature Transducer parameters(actually learn conventional speaker parameters)
3. Master handbook of Acoustics by F. Alton Everest.
Level Advanced
1. Building IEM experience
2. Tube acoustic physics(open, closed, variable dimension etc)
3. Resonance calculation
4. Method of representing the acoustic pathway as an electrical circuit and control individual parameters.
Level Extreme
1. Computer simulation of sound parameters and fixing it
2. Instrumental simulation of electrical parameters and fixing it(impedance and phase)
Level Godly(and I mean I haven't covered topics myself thoroughly)
1. Tube flow mechanics and Natural crossovers
2. Tia Concept(its really very hard to implement, but understanding concept thoroughly might help)
3. Helmholtz Resonator
4. Janus Resonator
4. Adjacent tube resonator
5. Hybrid pressure to wave change
6. Mapping heat zone inside acoustic pathways and fixing it with mathematical equations
7. Reducing distortion just by structural acoustic science.