<p>From 1994 to 2018, ISBN was part of Polish VAT law, acting as a requirement for the reduced tax rate for books. In 2019, this requirement was abolished. However, the Polish Book Chamber (Polska Izba Książki), a Polish self-regulatory organization, has been lobbying for maintaining it. The organization argues that, due to the problems with defining what constitutes a book, such a solution is beneficial, as the ISBN unambiguously indicates the “book status.” This article argues that such a solution is in fact flawed and policymakers should seek other ways to improve tax law, or any other law related to books. First, an analysis of the ISBN standard is conducted, which shows that the process of ISBN assignment is arbitrary and left to the publisher’s discretion. Then, Polish case law is analyzed, demonstrating that ISBN was not considered to be a reliable aid for the tax authorities and courts. Lastly, consideration is given to the question whether such a solution violates the fiscal neutrality principle of taxation and should be applied within the EU.</p>