This has bugged me for a while, and I can’t find the answer on the net. On the bottom of library books (if you hold it with the spine vertical), there are always highlighter dots, pen dots, marks of all colors. These marks aren’t found on the pages inside the book, but just at the base. It seems like older books have more dots. Why? How do they get there? It can’t be some sort of filing system.
I’ve worked in various libraries, and I’ve not seen anything like that. What you will often find in library books are:
Call numbers (from some classification scheme like Dewey of Library of Congress).
Bar codes for an automated circulation system.
Library stamps with the name of the library, in various positions.
An acquisition number or code, usually written unobtrusively in pencil on the inside edge of the title leaf.
As a historical relic, in some older boks in some libraries, a pencil mark underlining where the main entry started. (And I won’t explain main entry, except to say it is usually the author’s name or the title of the book).
But dots I’ve not heard of. What library, or kind of library, did the books come from?
Maybe it’s the remnants of something that was written across the bases of a stack of books, when tied or otherwise positioned together?
I’m seeing it here in Burnaby. And it’s driving me nuts. Mostly looks like marker, not highlighter. I just assumed it was bored and destructive kids, but obviously not if it’s going on elsewhere. I mostly bought books before, so I haven’t noticed it from other libraries. Remind me again how it’s better for my budget, and better for the trees, and all sorts of other good things for me use the library and not buy my books. Please. Cause library books can have some incredibly gross things going on in them.
Just a thought - why not ask the librarian?
Some answers from another board:
It’s surely the result of the book being transported in a backpack with loose pens and highlighters that mark up the book on the only exposed spot (the top and bottom of the book) as the bag moves around.
because people usually ahve pens in the bottom of their bags. when they put their books in the bag, the pens mark the books.
I think this is right. I see the marks on library books everywhere, these are not intentional marks (as in graffiti or long pen marks) but pen dots.
What occured to me, but IANAL (L being Librarian in this case) was that it might be that some library staffer might mark the bottom of a book with an easily recognizable mark when inventorying new books, to make sure each book only gets counted once. This wouldn’t necessarily happen at all (or even many) libraries, and might only happen with a particular person at a library.
On a somewhat unrelated note, I have known people to write their names on the bottom of the books, so when you’d look at the edges of the pages in a closed book, it might say “Andy” or something like that to identify in a non-distracting way who the book belongs to. Then of course, there’s the famous flip-book animation seen in the bottom corners of many a boring textbook.
Aha, I know the answer to this one, since I used to live in Burnaby and use BPL quite often. They used to mark a dot on the bottom of the book each time it circulated, using a different colour for each year. This way, when they were going through the collection to see which books to discard, they could see at a glance how often the book had circulated recently. I’m not sure if they still do this or not, but when I lived there, I got curious enough to ask a staff member about it.
I used to work for a publishing company (I still do actually, although indirectly now); we used to mark free sample copies in this way (as well as gratis copies provided to libraries) so that we might have some chance of becoming aware if they were illicitly finding their way to retail sale. I don’t think it ever actually produced a result, but we did it all the same.
I would think that the library wouldn’t be marking the books anymore when they get checked out if they’re automated. Do the newer books still have the dots?
I’ve also seen marker marks on the bottom of books that are in the remainders stacks at bookstores, so I assume it has something to do with pricing.
Another source of strange markings in/on books (IANAlibrarian, but I’ve volunteered at the local library for a many years) is patrons. 
Most often this is done with books by prolific authors that a reader has ‘discovered’ after the author has been publishing a long time, and especially for authors who do series with a set pattern of titling. The idea being to make it possible for the reader to quickly identify which books he’s already read, so he doesn’t accidentally take out “The Case of the Curious Canary” a second time.
Back in the dark ages of my youth, the library card* would serve that function. If your name was already on the card, you’d know not to take the book again.
For Harry Potter (well, the “then” Harry Potter) I imagine that wouldn’t work due to popularity, though. So: I don’t remember – I always went for Jules Verne and Robert Louis Stephenson and Isaac Asimov but not that hack Robert Heinlein (who I rather enjoy, now). So, what would have been the Harry Potter-level famous book in 85 or so?
*What’s a library card? Well, at the the school library, there were pockets with ruled index cards in the front cover. The card had the catalogue information for the book, so you’d know which card belonged to each book. You’d write your name on the next available line of the card, and the library worker would take it and file it by due-date. Of course with the county library, they used a slight variation of this system that preserved your anonymity, just in case you didn’t want your teachers to know you were reading Stephen King filfth.
Some of our new books have these marks - I AM a librarian, but not an acquisitions librarian. I always figured they were possibly remaindered (remaindered books are often marked that way, as somebody already mentioned) or marked as “library copies” by the seller, or something. (It could also mean “already RFIDed”, but I don’t think it does.) Anyway, I can testify that there’s definately books that have never been in a backpack or read by a “collector” that have marks on them like that.
They are called “remainder” marks.
It is explained at this bookseller’s blog
Go to this url then click on:
Why does my book have a pen mark across bottom page edges