No. I'm not even sure I understand what you mean. Nobody would describe an open book by saying the pages were at 90 degrees. I'm trying to picture in my mind what you want to describe but I'm still having difficulties. Do you mean the books look like this? http://www.openbookpublishers.com/shopimages/sections/normal/Open-Bookpic.jpg

These books where open at 90 degrees. http://www.openbookpublishers.com/shopimages/sections/normal/Open-Bookpic.jpg As Paul said, unusual to see a sentence like that.