I just came across this event while reading French magazines. I think it's an event that brings books to young people in France. I just can't find the translation of this phrase...is it like getting lost in a book? "leaving" in a book and not being able to put it down? Or is it more like leaving a book behind or something?
Note: I do not really know the event "Partir en Livre" so my presentation is shortened and necessarily inaccurate.
It is an event promoted by the French Ministry of Culture to encourage young people to read (c.f. the "official" website). Book professionals and associations meet young people to encourage them to read. They may move books to the beach, to the campsite... and do animations.
In my mind, the play on words means "imagine something" or "escape by the thought" through reading. The pun is derived from the collocation "partir en voyage", this event was also named "lire en short" (because people wear shorts during summer vacations).
Éditorial du ministre de la Culture [...]
Alors comme les petits héros de Zep, du 10 au 21 juillet, chevauchez des livres, envolez-vous et rêvez ! Bonne fête à toutes et à tous !
ministre de la Culture