[lifthrasiir]

I specifically said "lexicalized" because the word "wordbook" would be somehow related to "word" and "book" but can mean something else so these words can't no longer stand as a standalone word. Whether the word "thesaurus" can be said to be a "word book" or not is pretty much irrelevant here. If a "wordbook" meant a thesaurus then a plain dictionary would naturally use a different word (or phrase) to avoid confusion. Or more likely, "thesaurus" may not exist as a single word but would be just called a synonym dictionary.