Translation:The writer won in the novels category.
"kategorio" and "romanoj" don't seem to have a formal connection. Why not "kategoriaj romanoj" or "kategorio de romanoj" ?
romanaj kategorio is wrong, as kategorio is singular, the adjective modifying it cannot be plural
you could say "romana kategorio", but "kategorio romanoj" is better. in this case, its like saying "the category, that is called 'romanoj'"
I know we can put two nouns together in English so that the first one becomes as an adjective, but it is a very new concept to put two nouns together in Esperanto, especially the second one is a plural...... could you give more examples that we can use them more in everyday life?