Over the years I have spent a lot of money on educational textbooks. They are not cheap!
If you are a serious audio engineer or someone who loves a deep dive into the technical aspect of acoustics, then the following books are great resources.
Some of these books are speaker and headphone design specific, while others cover the broader field of acoustics in general.
Here is a list of my top books for each aspect of the audio industry from loudspeaker design to live sound engineering.