J.R.R. Tolkien, The Lord of the Rings. I re-read this every few years, and have officially lost count of how many times I've read it.
Maurice Keen, ed. Medieval Warfare: A History. Often I tell people that I became a librarian because I couldn't pick just one academic discipline to specialize in. I have an amateur's interest in medieval history, so I am reading this book.
Alistair Black, Dave Muddiman and Helen Plant. The Early Information Society: Information Management in Britain before the Computer. But of course, library science is a field of its own. One of the most interesting things about this book is its demonstration that current problems of information organization, volume, access, and authority aren't new. They predate the digital age by several decades. Only the manifestations of these problems are new.