"Sometime, Somewhere" is also from the vocal album (track 6 there) so don't worry, bring your translation back for that one track (goddamntherhyme).
Just to add it's not only localized as Mysterious Book, the Japanese premium editions of the game carried the same title in English (before any localization of course).
http://ecx.images-amazon.com/images/...L._SL1411_.jpg
Gust seems to be staying with the mysterious theme seeing the next game (Atelier Firis) will probably be "Mysterious Journey" in English.