I was helping my mother go through some of their old books, to donate to charity. I picked up one book, and a little glassine envelope fell out. Inside the envelope was a curl of hair. My mother looked at it and said, “Oh, that was from your first hair cut.” It had been in that book over 55 years.
Knowledge
yes it has a general definition, but within book collecting it has the specific meaning I used.
Even within book collectors, ephemera means things like posters and postcards and letters, and not necessarily just those which are found in books. Now, if there’s a letter or even a note in a book, from significant parties, yes, that increases the value of the bundle. I am minded of the instance of a cocktail napkin that was signed by an author at a speaking engagement, which was slipped into a book written by that author, along with a photo of the author, also taken at that engagement. The book itself was inscribed, as well, so the bundle was worth considerably more than an inscribed volume.
I cataloged books for a while, and was associated with a used/rare book store, and I went to the professional conventions. None of the people at those conventions used “ephemera” to mean only “book related ephemera”. They used it to mean “anything which is of a transitory nature”. Although that particular store didn’t deal in ephemera by itself, it only dealt with the things that were related to books that they sold.
Just the other day I recovered my copy of One Flew Over the Cuckoo’s Nest that a friend had at his apartment, a book that I hadn’t seen for 20 years. I know it was mine because it had an old credit card receipt that I had signed in 1990. Not very interesting I know, but it was kind of cool to see what my signature looked like way back then.
I wish I remembered the book, but I once found a bookmark - at least, it was the exact size and shape of a normal bookmark - that was pre-printed with graphic gay male porn pictures. Full-on penetration shots. No words. If it wasn’t intended for use as a bookmark, I’d be surprised, as it was simply the perfect bookmark dimensions.
Joe
As a kid, in a used copy of a “Choose Your Own Adventure” book I found an ant.
This poor ant had been squished between pages. It was easily twice its normal surface area though only a fraction of its normal volume.
Grossed me out huge. I have a minor phobia of insects and this didn’t help matters.
Once I checked out a copy of Terry Pratchett’s Lords and Ladies from the library. Inside I found copy of a very detailed painting of a typical fantasy scene. It had a wizard, some warriors, a couple dwarves, and an elf going on a quest through a forest. It was clearly not related to the Pratchett book. I can only assume that some other fantasy geek had checked the book out previously and left the painting in there.
I picked up a very sectarian book from the public library. This was a book published by the in-house publisher of a religious group that is widely considered to be a “cult”, and the book obviously was a form of evangelism, quite possibly placed there by local followers. Someone had added marginal refutations or accusations of half-truth-telling, and recommended by name a specific “anti” book, telling the reader to go find that “anti” book.
I had bought a bunch of Stephen Jay Gould’s books at Half Price Books, and when I started reading “Hen’s Teeth and Horse’s Toes” on the dedication page was written, in pencil, “View of how a 20th century Darwinian evolutionist approaches his subject.” The table of contents page page had checks and numbers written next to most of the chapter titles. Throughout the book there were underlinings and occasional circling, almost all of them of items regarding evolution. There were also occasional marginal notes; one part that talked about how marine fossils are often found high in mountains and far from the sea had “The flood” written next to it, and next to a sentence about how human beings evolved from apelike ancestors was written “I’m not from a monkey or ape!”
Can’t find it on my shelves right now, but I once bought a fairly recent world travel guidebook from the library for ~4$. After I got it home and was flipping through it, I found something like 80$ behind the dust cover!
There was a note written there to the effect of “Good luck on your trip”, etc., but no names, so I kept it. 
I can only imagine that it was an unappreciated gift to someone about to go on a long-planned trip, who ditched it at the library donation box. Anyone getting just a boring book from that lame aunt, check for $$$! 
I will sometimes make a small annotation in a margin in a book. Last time was about 20 years ago when I was reading a library book about Jack the Ripper. The author mentioned something like one of the women was in an area which the future Edward VII frequented. A chapter or so later he asserted that she was pregnant and Edward VII was almost certainly the father.
I wrote something like “Tripe. You haven’t shown they even met”. Other than that I don’t get very emotional about them.
I bought some used books from a shop in the North when I lived up there and later found a little bit of money between the pages. Not much, enough for a few beers, but it was a pleasant surprise.
Someone brought a box of old books into my store. In one of the books was a portrait of a good friend of mine with his (now ex-) wife and their baby, who is now in her twenties. The odd thing is that my friend didn’t remember them ever owning that book, nor did he know the person that brought in the box.
One of the guys at work showed me a paperback he had bought from a second hand store. Inside were five new $1 notes- all crisp and everything.
Australia hasn’t used $1 notes for something like 20 years.
I collect Richard Halliburton books. In one of them I found a postcard from the early part of the 20th Century. It was a group photo of some of the Reelfoot Lake Night Riders. Who were those masked men? They were locals who lived in Northwest Tennessee.
When land developers moved in and tried to profit from the formation (almost 100 years earlier) of Reelfoot Lake, the locals didn’t cotton to them. the argument over who should have access to the earthquake formed lake had been going on since about 1812 or so. The locals thought that it should be open for fishing to everyone. The developers thought that it should be private.
The Night Riders eventually hanged one of the lawyers for the developers. Another lawyer got away by swimming across the lake. Some of the Night Riders were tried and convicted of murder in Circuit Court, but that verdict was overturned by the Tennessee Supreme Court.
The postcard that I have is the only photo that I have ever seen of them. This particular book may be one that I got from my grandfather.
The masks are very much like the full face masks I sometimes see Tennessee Titans wearing on the sidelines during the winter.
Think I will go digging through my Halliburton books to find it now…
I’ve found scores of receipts and other boring things like that.
My most interesting find at a garage sale was a diary of a priest from the 30’s or maybe the 20’s. I’ve never read all the way through it, only the first 10 pages or so, as there is no OMG stuff going on. I think I put it in the safe, I haven’t seen it in awhile.