Well, there's Richard Seaford's 2006 book Dionysos, in the Routledge 'Gods and Heroes of the Ancient World' series. For what you are asking, it seems the clear winner.

I'm a little hesitant to suggest it, though. Not because it isn't good -- it is, Seaford is a top-flight scholar -- but because of the price. Converting from my currency the price on the Routledge website works out to about €31. Amazon.fr is even worse at €38. It's a good book, but I don't know that it's €30-40 good. Especially when it's a 150-page paperback, aimed at a general audience.

Aside from Seaford, there are two scholarly options that you're likely to see pop out when you search bookshop websites.

• Marcel Detienne's Dionysos mis a mort (1977): Detienne is one of the foremost French-language scholars of Greek myth of the 20th century. But if you want to read his book, I hope you like structuralism. Also, the book is about Dionysus in Orphic/mystery religion, not Dionysus in general.

• Walter Otto's Dionysos (1933), translated into English in 1965 as Dionysus. Myth and cult. Avoid, avoid, avoid: Otto is hopelessly outdated. In his time it was still thought that Dionysus wasn't a 'true Olympian', because the ancient Greeks themselves believed Dionysus was a recent import from Thrace: since the 1960s we've known that's absolutely wrong, and Dionysus is among the oldest, most central gods in the Greek pantheon. (For reference, the same idea coloured the interpretations of people like Nietzsche too.)

Seaford really is the best option. If your research needs have a lower budget, I guess you could simply open up a reliable encyclopaedia of myth -- but that generally means focusing on literary myths, rather than religion. The exception is the Roscher Ausführliches Lexikon, but of course that isn't in English or French.

So, I'm sorry for your bank account, but I'm going to have to say that Seaford is not just your number one option, but quite possibly the only option! I think the fact that it's relatively recent is a major point in its favour too: even in the 70s, Detienne got flak in reviews for suggesting that Dionysus was a regular Olympian just like the other gods. That stigma had dissolved by the time of Seaford's book.