Hi Jean-Marc, welcome to Javaranch!
I have seen that the recommended book is "Just Java 2" in order to be prepared before the jump into the cattle drive experience. If I want to read "Head First Java" is that a problem regarding the content and tutorial of the cattle drive ? Or could I buy both of them?
You could buy as many as you can afford, and then a few, that's what us book addicts do.
As you go through the Cattle Drive, the (earlier) assignments give you specific pages to read in Just Java 2, but the material can just as easily be found in any beginner book. Head First Java is an excellent choice.
Not everyone who goes through the Cattle Drive uses Just Java 2, some don't even buy a book at all. It kind of depends on your background and your personal preferences.
Oh, and like it says in Head First Java: "There are no stupid questions."
Cheers,
Pauline