Feather Brain: Developing, Testing, & Improving Saltwater Fly Patterns by Drew Chicone
This is a must have book for fly tyers everywhere, REGARDLESS if you tie saltwater flies. If you tie trout streamers- you need this book. Bass flies? You  need this book!! Drew Chicone is a fly tying machine, and his process and methodology is so refined anyone who picks this up will find something useful. 

Want to dye your own feathers? Read this book. 
Want to increase your fly tying production methods? Read this book. 

Honestly, it's one of the 10 theory books every fly tyer should have in their library.