Although a dictionary is a book, the needs of a general-purpose dictionary is better served at our sister project, Wiktionary. Also:
Wikibooks is not a thesaurus. There is a thesaurus project at Wiktionary:WikiSaurus. Books that only give lists of synonyms or slang can be deleted from Wikibooks.
Exception: Wikibooks does allow and encourage projects that require supplemental dictionaries or thesauri, such as a glossary. For example, a mathematical textbook might need a glossary of math terms; a foreign language textbook might include an English-to-foreign and foreign-to-English dictionary. For these dictionaries and thesauri to remain, they must be secondary and auxiliary to a main text.