Have a look at the Home Library Mission Statement of Boston Bibliophile. It’s where she states what she believes the purpose of her personal library to be, and the ways she intends to treat it, to accomplish that purpose.
Now, I think sometimes we pigeon hole things a bit too much, and if we’re not careful we could control and codify to death something like our personal libraries. To some extent, I think they need to be organic and grow naturally.
And yet! You know how you sometimes go to your library and discover a whole bunch of books that make you wonder how they even got there — and of course, those are the easiest to purge if you feel you need to get rid of a few tomes. Having some kind of general idea of what you really want in your library, and how you’ll maintain it, can really help to keep you from ending up in such a situation.
I’ve got 2000+ books. Maybe I should mull over a mission statement of my own, or at least think about ways to approach and use my own library to greater personal advantage. Hm…