Bookseller's Labels -- A Vanished Custom 
Back in the good old days, some of your "better" booksellers, of both new and second-hand tomes, often affixed their own custom-printed labels inside the books they sold.   These were usually quite tiny (like mini-bookplates) -- rarely bigger than a postage stamp -- and were customarily placed in an unobtrusive spot, most typically at the bottom corner (nearest the spine) of either the inside front or inside rear cover (the pastedown, in book trade lingo).  This was by no means a universal practice, or even an especially widespread one -- it's possible to look through dozens or even hundreds of vintage books without discovering a single label of this sort.  But that just adds to the twinge of delight that finding one almost invariably brings. 
I think the pleasure I take from these little things has something to do with a certain dimensionality they add to the mostly-unknown story of a particular book's previous life.  To buy a book unadorned with one of these is, often, to simply buy an "old book"; from the evidentiary front matter, one can usually divine that it was published by this or that company, in a particular year, and so what?  But the specificity of knowing that it spent some time -- perhaps was sold for the very first time -- at the Satyr Book Shop (on Vine Street in Hollywood, California) or The Book Shelf (in The Doctors' Building) of Cincinnati, Ohio, adds a nice geographical element to its journey to your shelves.  (Previous owner's inscriptions are often good for this as well, and have their own charm -- but give me a vintage bookstore label any day!) 
Here are some of my favorites -- more to be added as I discover (or rediscover) them.  Generally speaking, I've enlarged them in the scanning process.