This is the list I'd recommend:
Rafael C. Gonzalez, Richard E. Woods - Digital Image Processing
Great introductory book. Well written, a lot of examples. Though it is not deep in any of the fields.
Alan C. Bovik - The Essential Guide to Image Processing
A comprehensive book on many image processing related subjects. Gret book to skim through.
A fundamental book on image processing for electrical engineers is
Two-Dimensional Signal and Image Proccesing_Jae S. Lim
A highly recommended one, again, for electrical engineers is
Fundamentals of Digital Image Processing_Anil Jain
A hands-on book on basic practical image procesing is
Principles of Digital Image Processing_Wilhelm Burger