Sunday's New York Times magazine has a delightfully meditative piece by Henry Alford, called "You Never Know What You'll Find in a Book."
Alford talks about the slice of fried bacon found in a volume in the Duke University Library, and a letter in a used paperback which read, "Do not write to me as Gail Edwards. They know me as Andrea Smith here." There's a novel lurking in that idea -- and another one in the rejection letters that a miffed author inserted between the pages of several copies of the book after it was published, and then sold to a used book dealer.
In my own experience, I have found library books to be an interesting source of between-the-pages litter. Receipts, letters, addressed envelopes, and photographs have all obviously been used as bookmarks, and then forgotten. A box on the counter of one branch library is there especially for people to leave such things.
I must confess to being a deliberate between-the-pages stuffer myself. Originally, I was following the example of Fildes, a prominent early Wellington book collector, whose collection is now in the Beaglehole Library at Victoria University. All the books he owned have a special added interest, in that he inserted newspaper clippings (too often, alas, unsourced and undated) relating to the content, in each the book, along with bits of paper with scribbled comments about said content, sometimes quite unkind. The receipt from the bookseller from whom he bought the book is often there, too, recording a fascinatingly small amount paid for a volume that is now quite valuable.
So, with my own books, I started slipping in relevant newspaper cuttings (carefully sourced and dated, of course). Then I started adding relevant letters I'd received, sometimes from the author of the book, and occasionally from people who are researching the same topic. This has proved frustrating in the long run, as there are times I need to find a certain letter, and can't remember which of the books I put it in. But it is tremendous fun when I buy a secondhand book and find that the previous owner(s) did exactly the same thing.
The most exciting discovery came the day a copy of Nimrod of the Sea, or The American Whaleman, by William M. Davis (Boston: Charles E. Lauriat, 1926) arrived. The first owner had signed his name and date, "P. B. Blanchard, 1926," on the flyleaf -- and I "knew" him! I quote from my book about the strange lives of seafaring wives of captains, Hen Frigates, chapter one.
On October 3, 1906, twenty-year-old Georgia Maria Gilkey of Searsport, Maine, was married in her graduation dress. There had been no time to make a wdding gown, for the bridegroom was a seaman. Captain Phineas Banning Blanchard had proposed to her during one of his fleeting trips home, and one week later they were married. Georgia felt no doubts about the headlong courtship. As she reminsced later, when Banning had taken her out sleighing the sled had capsized, dumping them both in the snow. And that, according to a local old wives' tale, was a sure sign they were to be wed. So George married her captain in her gradulation gown, carrying a bouquet of pink carnations. And, after a hasty buffet luncheon, the newly weds took the train to Philadelphia, to embark on the great square-rigger Bangalore, for a honeymoon voyage around Cape Horn.
Not only had Captain Blanchard signed his copy of Nimrod of the Sea, but he had used a picture postcard of the Bangalore as a bookmark, which was there in the book -- and is still there now.