Why did Henry's novel become a bestseller? Well, of course it did - Henry's curse still applies, he'd likely have no trouble whatsoever becoming a bestselling author.
What's more, Addie knew that would happen. In the end, the design of Henry's curse was a mistake by Luc, just not the one Addie originally thought.
So Addie immediately recognizes that Henry's curse, to be exactly what people want him to be, is why Henry is immune to Addie's curse, as what she wanted most was to be remembered. But why was Addie immune to Henry's curse, as we later see from Henry's perspective? Addie doesn't think she is, but in truth, Luc just specifically made them exceptions to each other.
Actually, the issue with the curse is not that. During one discussion with Adeline, Luc said that "I have no power over promised souls". That would make the true reason that she is immune to the spell be because Luc's curse does not have the ability to impact anyone else who made a deal with him. As Henry said, there was no frost in her eyes. In other words, if Adeline was on an island with everyone on the island having made a deal with Luc, then everyone on the island would be able to remember her.
Luc one time says to Addie that he can sense her heart and can know when it falters. Each time Luc takes a soul from a person the person is alive. I think that this means that Luc can sense when the people who have deals with them are nearing death so that he can claim his part of the deal before they pass on.
Fridge Logic:
How does Addie spend whole days with people throughout history? Carefully avoiding leaving their line of sight works in most cases, but one presumes some of these historical paramours would, for instance, go to the bathroom at some point.
