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)