Some adjectives can also be used as nouns :
a) to denote an abstract quality - the theatre of the absurd ; the good of the environment; the realm of the unknown.
Adjectival nouns describing abstract qualities are grammatically singular : The unknown is scary!
b) to denote a class of people - the rich, the poor, the unemployed, the overweight, the Left. Nationality adjectives ending in /s/ /z/ /ʧ/ or /ʃ/ may often be used in this way : the British, the Chinese, the Danish, the Dutch, the English, the French, the Irish, the Japanese, the Swiss, the Welsh, etc.
Adjectival nouns describing a class of people are grammatically plural : Information on who the unemployed are—and who they are not—can provide valuable information about...
Want to improve your understanding of language?
Thornbury, S. About Language, Cambridge
(Remember that Amazon will often have used copies of the books you want which are much cheaper than the advertised price. It's always worth checking.)
