Fallacy can be used as a mass noun to refer to faulty reasoning in general or as a count noun to refer to a single failure in reasoning (or multiple instances if you are using the plural).
If you are going to use the plural, be sure to check your verb for agreement:
There were a lot of fallacies in the book
I think that it is OK to use the mass noun and that it is a matter of style whether you choose one form over the other.
There was a lot of fallacy in the book.
The book was full of fallacy.
If you are referring to a specific fallacy, like pathetic fallacy you shouldn't use the mass noun. As @TRomano mentioned, you should say something like
There were many examples of pathetic fallacy in the book.
The book is filled with pathetic fallacies.