A single book? I don't think it's possible. I have a couple shelves full of good books that are good resources for various subjects, all related to some aspect patient care from assessment through rehab.
Although it's not a textbook per se, The 60 Second EMT is a book that will help point a new EMT towards noticing things that are important about patient care. It helps integrate the textbook knowledge with what you're going to see in the field. It doesn't replace good FTO time, so think of it as a "head's up" type of resource.
I think a better question to ask would be: If you could recommend one book for every stage in an EMT's education, what would they be, and for when?
Somehow, I think the end result would be a pretty good list, that when strung together, results in basically a good science foundation that would be appropriate for most allied health programs.