Is dictionary a synonym for book?

No, "dictionary" is not a synonym for "book." While a dictionary is a type of book, it has a very specific meaning.

Dictionary: A reference book containing a list of words in alphabetical order, with their meanings, pronunciations, and often etymologies.

Book: A written or printed work consisting of pages bound together and containing text, pictures, or both.

So, all dictionaries are books, but not all books are dictionaries.