The Dorothy L Sayers Society published an encyclopedia-type reference book back in 1985 in which one could find the origins of those elusive quotations from poetry, translations of Latin phrases, definitions of slang, photos and maps of locations from the Lord Peter Wimsey mysteries. The Lord Peter Wimsey Companion, edited by Stephan P Clarke, is of great usefulness to Wimsey fans who are not as well educated as the author, and few of us can claim to be the equals of Sayers (or Wimsey) in that regard.
In 2002 the most recent printing of the 2nd edition was published and I bought a copy for something like $25 from the society, to which I belonged at the time. I little realized until I owned it just how valuable it would be.
The maps of Oxford and plans of the colleges there are invaluable. There are illustrations, line drawings and photos, of London locations, the Wimsey family home, the gallery and beams of the church in The Nine Taylors. There are maps of England and London in the early 20th century and one of the western front in World War I. There is an excellent family tree going back to Roger de Guimsey who arrived in England with the Conqueror in 1066, a floor plan of Pym's advertising agency, photos of the houses in which Peter, Harriet, and others lived and of the Belona Club (in real life the Army Navy Club), diagrams explaining how bells and change ringing work. A photo of the church where Peter and Harriet were married. A line drawing of Talboys. And much, much more.
The bad news is the book is and has been out of print for some time and the copy I have (fine/fine) now has an asking price at abebooks of $600. A paperback of the first edition can be had for less, but still it's an expensive book to acquire these days.
The good news is that the Dorothy L Sayers Society has published a 3rd edition online and it is available to members of the society (the patroness of which is P D James - or does one call her Dame Phyllis?) The cost of membership is a little high, 44 dollars in the US, 18 pounds in the UK. But the society does many good things (a peal of bells on Lord Peter's 100th birthday, the restoration of D L Sayers' home, publication of many other worthwhile books, including the complete collection of the author's letters.)
There's more information at the society's web site.