This is a large topic... I'm not sure what you're after when you say processing. As for construction, you would do well by looking for the articles in the electroacoustic literature. You should start with Poldy's chapter in Borwick's "Loudspeaker and Headphone Handbook". Poldy also has an introduction to headphones in the AES library. This is just the tip of the iceberg...
I am looking for a handbook about headphones, describing all technical aspects for construction and digital audio processing. Do you have any suggestions?