Cecile Dubuis wrote a master's dissertation for University College London titled "Libraries & The Occult." I've only read bits of it, but the challenge she identifies is that occult books are, by their nature, anomalous and hard to categorize, much like the phenomena discussed in their pages. As a result, they are often unsearchable in the context of traditional library classification systems. From the dissertation:
The occult seems to be one of the least considered subjects when it comes to classification. This can often result in materials being divided among other subjects such as philosophy, psychology and religion. This can make it difficult to find occult materials. In such cases, a further difficulty can arise for the user; that of asking for help in locating "occult books". …reactions may not always be negative or judgemental but it does depend somewhat on the beliefs and opinions of the library staff concerned. Particularly those who eschew the subject from a standpoint of little or no personal knowledge…
This subject is ambiguous and marginal in virtually all ways: socially, intellectually, academically, religiously, scientifically, and conceptually. It does not fit in the rational world but this is also what makes it so fascinating and interesting. The imbalance between the amount of interest in the field and the stock within the library system is a result of such dilemmas.
Link (via Further: Strange Attractor & Beyond)
