All it takes is a simple S to make most English words plural. But it hasn't always worked that way (and there are, of course, exceptions). John McWhorter looks back to the good old days when English was newly split from German -- and books, names and eggs were beek, namen and eggru!
A thesaurus is a reference which allows a person to look up synonyms, and sometimes antonyms, for words or phrases. It comes from the Greek thēsauros, which meant a treasury. The adverbial derivative is thesaural.