1. A method for processing image data read from a book document placed on a platen, the method comprising: determining a target pixel of the image data as a background pixel when a difference between the target pixel and a neighboring pixel is less than a threshold (step 1 ); performing a preliminary luminance correction calculation using a same correction value candidate for a luminance value of a background pixel positioned on a line parallel to a binding portion of the book document (step 2 ); integrating an absolute value of a difference between a value obtained by the preliminary luminance correction calculation and a luminance value of a neighboring background pixel positioned at one side of the line with respect to background pixels on the line, and selecting a correction value candidate that minimizes the integrated value among a plurality of correction value candidates as a correction value of the background pixel on the line (step 3 ); calculating a cumulative luminance correction value for each line by successively performing a luminance correction calculation in neighboring order of the line based on the selected correction value (step 4 ); extracting a shadow region of at least one of a shadow of a binding portion or a shadow of a front edge portion involved in the image data based on the cumulative luminance correction value (step 5 ); and performing a luminance correction on the shadow region based on the cumulative luminance correction value (step 6 ).