I see nothing wrong in having a guide with you somewhere in the truck.
How many OB calls do you go on in a year? Wouldn't it be nice to have something when you get dispatched on a maternity, to just look over and refresh your memory. There is lots to learn and know in EMS. You learn by repeating it over and over the right way. My service doesn't run OB calls very often at all. So yes, its a weak spot of mine. I could sit and study it once a week or something, but with out the practice will I retain it all?
Use the field guide as your pre-flight checklist.