An "Open-book” pelvic fracture suggests a diastasis and/or a fracture of the pubic rami with a posterior pelvic disruption of the sacro-iliac joint. http://www.springerlink.com/content/y376746r2458q663/

Marvin Tile, David Helfet, James Kellam
Fractures of the pelvis and acetabulum, 2003, p 173

There is diastasis of the symphysis pubis and associated diastasis of the right sacro-iliac joint. There is also a subtle fracture of the right inferior pubic ramus. The sacral arcuate lines (sacral foramina) appear intact. Potentially useful supplementary views would include pubic ramus views and Judet's views. No supplementary plain film views were performed- a CT pelvis was performed in lieu of further plain film imaging. In addition to the fractures/dislocations identified on the plain film pelvis image, a comminuted anterior column fracture of the right acetabulum was demonstrated on CT imaging. It is not uncommon for CT imaging of pelvis trauma to reveal new fractures or further information about fractures which have already been identified on plain film imaging. This type of fracture is termed an open book fracture.