You know how to get the hypnosis book first just curiousAtelle disappearing for long periods of time is not normal. She should always be somewhere on the map, either available for interactions or in the middle of a NTR event.
If she's disappearing, there's a bug happening. The only reliable way I've seen to trigger this is to take the hypnosis book from the library, preventing Locke from learning it. That fucks the game greatly. I think there are some other ways to trigger it, but I haven't experimented anywhere near enough to figure them out.