The item you were looking for has been culled from the collection. Items are culled for various reasons. More important is the question of what to include in the collection, which thus informs what to remove.
Materials are part of the collection because they are things we are sure we will want to re-read, we know we want to read before we die, or we have some emotional or aesthetic reason to keep them available.
Another possibility, especially for an esoteric archive such as The Hermetic Library, is that this item was under the seal of a particular degree and unpublished.