Wikibooks is already multilingual you say. Wikibooks is offered in over 100 languages. However, a new multilingual Wikibooks project has been proposed on meta that would act as a staging ground for multilingual work and coordination.
Think of a project where:
#Book translation projects could be coordinated
#Multilingual books, such as books on how to speak one language written in another, could be produced (or edited)
#Books could be donated and converted into wikitext (and then possibly translated into many languages)
Where commons is a multilingual media project, this proposal could be a multilingual book project. There is a growing demonstration of this new multilingual Wikibooks on the incubator wiki. It's an interesting idea that I hope gets some good attention in the next few weeks/months.