First, 口語 is a noun, and この本は口語です doesn't make much sense. You usually have to say "この本は口語で書かれています" or "この本は口語体で書かれています". Likewise, この本は料理の本です is OK but この本は料理です is not. (If one has to rapidly sort hundreds of books about various topics, he may just say 「この本は料理、この本は動物、...」, but that's an exception)
Second, you may have to rethink 口語 is the right word, because 口語 (and its antonym 文語) is an ambiguous term. 口語 may refer to:
- The style of Japanese mainly used in modern (casual) conversations, as opposed to the style used typically in official documents and news articles.
- Modern Japanese in general, as opposed to classical Japanese.
- 我、その本を読みけり。 : Unequivocally 文語
- 私はその本を読みました。 : 文語 or 口語 depending on the context
- 俺、その本読んだよ。 : Unequivocally 口語
In other words, one can say that more than 99.9% of the books newly published today are in 口語 (in the second sense). This confusion is described elsewhere.
If what you want to mean is the first sense (conversational Japanese), the safer phrases are 話し言葉, 会話のような文体, etc.