Alan, I recently did a book and it was great to have to crop certain photos in order to get them into the format I wanted. Mine is a landscape book and I did a combination of horizontal images, vertical images, and two-page spread images. I also included some square images. The two-page spread images bled off all sides of the page, so I had to crop them to the exact size of the book. Some pages were full bleeds of one image per page which meant cropping to a different ratio, of course. I wound up liking some of the crops better than the originals I had worked up. It forced me to take second looks at a lot of the photos and reorient my thinking about what they would feel like and how I could best trim them. You have many options. You can vary your sizes and shapes or keep them consistent. I was mostly consistent but there were several variations throughout, which help punctuate and energize the viewing experience. Be conscious of rhythm when you compose the book. People turn the pages and there is a definite rhythm that you can help create with your juxtapositions and with what happens on the turn of each page. I included just a couple of black and whites, but most of my photos were in color, for this book. Other books I might do would probably have a lot more black and white.