The British Library in London houses the Library's main reference collections, spanning manuscripts and printed books from all periods and all over the world, as well as outstanding collections of maps, music philatelic materials, patents, and sound recordings. In addition to published catalogues, a number of unpublished finding aids, including some quite substantial supplementary catalogues and indexes, have been developed over the years. This useful guide gives the locations of these often little-known finding aids, their availability, and the extent and nature of the information held, providing valuable keys to further research among the collections.