This is by way of advice: you don't need to buy this book if you think what follows is a likely guide for you.
• Sometimes it's not possible to use a photograph on a double spread without losing something important in the gutter, i.e. down the crack between the pages. Cutting the photo in half and adding padding, i.e. blank space of any colour you choose, to the sides that meets the crack can avoid the worst of the loss, but how much padding to use?
• This book was a test. My experience from handling the book that was delivered from Blurb's printers is that 8pt or 9pt padding each side of the crack does the job well; 8pt each side the crack for pages near the front and back of the book, 9pt each side for the pages closer to the centre of the book.
• For an ebook, where you don't want to see any blank strip down the middle, it's better to use the double spread layout offered by Lightroom's book module, which is the software I normally use for my books. The view of the double spread in the Preview here, is misleading except for the ebook version; in the print version a strip about 8mm wide is entirely lost in the crack.
• Because Blurb's books don't open flat, as much padding as possible is needed to see a whole double spread or one, full-page photo. It's not just that the binding that cuts out some of the photo but the last few millimetres of the 'visible' image are near to vertical when you look into the gutter, so taking care of what will otherwise be entirely lost seems really worthwhile.