I collect books whose titles are numbers. I was inspired to do this via my friend Sharon, who has a collection of anthropomorphic body parts, e.g. a stuffed kidney with a face, arms and legs. Her idea was that she wanted to collect something, but that she wanted that something to be a sort of something where there isn't too much out there to collect so that the collection doesn't get huge and unwieldy. I'm not sure she said this, but another aspect of it is that it is something which is not neatly cataloged for you already so that you're just checking off a list. This contrasts it with nearly all "normal" collecting, such as stamps, coins, trading cards, etc.
In my definition, a book counts if its title is a number stated in either numerals or words ("27" or "Nine", or even "The Golden Ratio"). It can be prefixed by "the", as in "The Eight", but can't have other non-number words in the title. I'll take very subtle variations like "'48", which means 1948. It can have a subtitle with non-numbers. Sometimes it's not easy to tell whether something is a subtitle or not. This can usually be resolved by looking at the copyright page, but if that fails, I'll go by how it looks on the shelf. I'm willing to take books whose titles weren't intended to be numbers, but could be interpreted that way. So if a book is titled "I", I'll take that to mean the square root of negative one even if the author meant the first person subject pronoun. (Which leads me to also pointing out that the numbers need not be real.)
As I already have one and a half shelves filled, I'm not sure I successfully picked a low volume collection object. Oh well. I mitigate this by only adding to the collection (except for gifts) when I happen to be in a used book shop where I find a book that fits on the shelf. I might go so far as to buy a new mass market paperback if I think it will be hard to get used, but won't spend more than that. I certainly don't go online and hunt down more books, as I think it's obvious that could get out of hand.
Currently, I have:
As you can see, it starts to get silly at the end there.