Matteo, the Art of Hellboy is a great book but I wouldn't worry too much because I suspect that a lot of the sketches at the back of the Hellboy Library will replicate the contents of the Art of... Most of the extra material in vol. 1 is contained in the Art of.
Besides that, I think Dark Horse will probably reprint it somewhere down the line like they did with the Art of Sin City when they did the Sin City Library project.
Once again, it is a great book but whether it is worth 100 smackeroonies depends on just how much you dig HB and Mignola.

Btw, I'm a big fan of the first film which IMHO is the most succesful screen adaptation of a comics property and I can't wait for the sequel.