As far as I know, the relative clause should be placed immediately in front of the noun it modifies. For example, in sentence [木の下で休んでいる人の眼鏡], relative clause [木の下で休んでいる] modifies the noun [人]. Then why in sentence [今使っている日本語の本] relative clause [今使っている] modifies noun [本] instead of [日本語]?
In the second case [今使っている] should modify [日本語] (the noun it stands in front of) and the full translation is going be something like "Book about the Japanese language that (someone) currently using". But I'm pretty sure that the correct translation is something like "Book about the Japanese language that (someone) currently using". (Bold points the word modified by relative clause)