You bring up a good point.
First, if all are in the book (fully perfect, without sin) then the teaching would be full blown Pelagianism. (
10 Things You Should Know about Pelagius and Pelagianism)
Second, which sin would be the action that would cause God to erase someone from the book?
Third, what human action would get someone back in to the book? (Some here would argue that you must repent and be baptized to get reinstated).
Fourth, what actions, after repentance could get you erased again?
Fifth, when would the cycle of repentance end and could one accidentally be in a moment of unrepentance that thus sends one to hell?
Finally, where has grace gone?