Must Have
Understanding Exposure, Bryan Peterson
Adobe Photoshop CC for Photographers, Martin Evening
The Digital Negative, Jeff Schewe
Recommended
The Digital Print, Jeff Schewe
Nik Software Captured, Tony L. Corbell & Joshua A. Haftel
Photoshop LAB Color, Dan Margulis
Creative Black & White, Harold Davis
Black & White Photography, Michael Freeman
There are many other worthy books out there. These are books that I would maintain in my library if they were the only ones I could keep.
LightCentric Photography Blog 