there is a dummy subject and can be either singular or plural, so for 'there is X', we must examine X to determine the plurality of the verb, as if books were the subject of the sentence.
Words such as 'none, any, all, more, most, and some' may take either singular or plural verbs, depending on the context.
In your example, the subject (there/book) is singular, so it should take a singular verb form:
- There isn't a book. CORRECT
- There isn't any book. CORRECT
Note: If the word 'books' is used, then the verb should be the plural form
- There aren't any books.