Mind the gap

While there's always considerable serendipity involved in book purchases, there are also those books which you know you should buy...and yet, somehow, never do.  A pre-1928 edition of  The Book of Common Prayer, for example.  Granted, it's easy enough to check a reference in a library copy or a hypertext edition.  But the book's physical absence from my shelves has nagged at my conscience for some time.   Recently, guilt finally overcame inertia, and I diverted my attention from obscure Victorian religious novelists long enough to invest in an edition distributed by the SPCK. 

What books are missing from your library?