Don't look just at -x. Nouns and adjectives ending in -al usually become -aux (cheval > chevaux, idéal > idéaux), with some exceptions like bijoux and feux. On the other hand there's not so few adjectives like the one above, ending in -eux and changing to -euse in feminine version (joyeux, heureux etc, with jaloux > jalouse as a special case)
Here are some examples:
Actually, Duolingo provides 'important' as a translation for 'sérieux', and the French dictionary also cites 'important' as a usage under its listing for 'sérieux'. I have a feeling that this translation may apply in some contexts. Can any naturalised speakers confirm this?
Well I'll be...so does my dictionary. Interestingly, it doesn't give sérieux under its listing for "important." Go figure.
From what I can glean from the entry, it can be translated as "important" precisely in those cases where "serious" could be substituted without changing the meaning, e.g. "It came as a surprise to some that Ross Perot was an important candidate for President." There may be a case for this sense applying to books, but I'm having a hard time coming up with an example this morning.