A very good book is: digital image processing using MatLab, and you can use the following link for functions and algorithms using MatLab

http://www.mathworks.com/matlabcentral/fileexchange/

See the below link. It will give you all the function for IMPORT/EXPORT/ WRITE/READ/ etc 

http://in.mathworks.com/help/images/functionlist.html

http://in.mathworks.com/help/images/functionlist.html

I have read that. But couldn't understand how to do that. I have a question if someone can answer.

Write a MATLAB function called countGrayLevels that accepts as input an image matrix called I, and v, a 1xN row vector of strictly increasing gray-level values. This function should return as output a 1x(N-1) row vector, y, where: y(i) is the total number of pixels in I that satisfy: v (i )≤I

You have to write a cycle through all elements of I image matrix, and compare every pixel's intencity with all v values.