In the book, there's a different set of rules involving the aliens. While they're still called mimics, instead of the Omegas and Alphas, there are Servers and Backups and Antennas, and if a person inherits the day-resetting power, they can't lose it with a blood transfusion. Without going into the complicated specifics about how looping works inAll You Need is Kill, we'll just skip to the ending, which reveals that Rita has become an "Antenna," which means the Mimics are still able to loop. After Keiji kills the Server, Rita lures him into a fight and reveals that Keiji has to kill her in order to end the loop. If he dies, it's forever. If she dies, the loop will end and so will the war eventually. Only one of them can escape either way. Keiji reluctantly fights back, beats her and kills her.
The book takes a much more tragic approach to the ending, which has Keiji emerging as a hero of the war. It's drastically different from Edge of Tomorrow in both respects, as the film's version allows Rita to keep her life but takes away Cage's glory. I get the impression Keiji would have been very willing to give up the medal he received when the war was over for a chance to have Rita back, so there is a trade-off, but it's definitely an uneven one.