Here are some things to consider:
* Manga: There are 38 volumes of the original Yu-Gi-Oh! manga, plus several spin-offs and side stories.
* Trading Card Game: There are thousands of individual cards released over the years. These are often grouped into sets, which could be considered "books" in a way.
* Novels: There are several Yu-Gi-Oh! novels, both based on the anime and with original storylines.
* Art Books: Official art books showcasing character designs, card art, and other artwork from the franchise exist.
So, there's no single number that definitively answers your question. It depends on what type of "book" you're looking for!