Currently I'm reading Digital Signal and Image Processing by Tamal Bose. It has image processing concepts at the end of most ordinary DSP sections, so it's an easy extension of what you already know. Plus it has three or four chapters on adaptive filters.
Digital Image Processing By Gonzalez is the best it includes the fundmentals of digital image processing
1- Introduction
2- Digital Image Fundmentals
3- Digital Image Enhancement in the Spatial Domain
4- Image Enhancement in the Frequency
5- Image Restoration
6- Color Image Processing
7- Wavelets and Multiresolution Processing
8- Image Compression
9- Morphological Image Processing
10- Image Segmentation
11- Representation and Description
12- Object Recognition
also there is another version by Gonzalez about using Matlab
Download the Digital Image Processing using Matlab by Gonzalez from the Ebook Upload/Download. This is good for beginner, and you can even practise it using matlab following the guide there.