We strongly advise against cropping images externally, as most photo book design tools have the facility to do a “virtual crop”. This means there is added flexibility to tesselate multiple images into a set of defined placeholder shapes, since in some cases, cropped images will not fit optimally into the relevant placeholder.
If adjusting colours, brightness, and contrast, do keep in mind that every monitor will show colours differently (unless properly calibrated to the output device). Also, as monitors are backlit, images will appear more vibrant than when printed on paper. We therefore advise caution when undertaking such work until you get a feel for the effect on the printed page in your photo book.