Synoniemenbüch, probably?

Synonymwörterbuch, apparently:

https://shop.duden.de/collections/worterbucher-und-sprache/p...

Synonym, of course, being latin.

Lazy just like us. Thesaurus in Romanian is "dicționar de sinonime". I won't bother translating it back to English, you can figure it out :-D

A thesaurus is a type of dictionary.

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.

Likewordbook, surely.