I've been reading a lot the last few days about why libraries don't purchase self-published books. Vanity presses are being discussed on Publib, and the Librarian In Black posted about the elitism and laziness of libraries that won't purchase these materials. I disagree with "elitism" but maybe not with "laziness." This is part of a larger issue of collection development. It is too often a reactionary process - librarians look at reviews being fed to us from a small number of review journals and order what the reviews recommend. It should be more than that: we should be examining the collection, determining what is missing, and then finding it. Otherwise, your collection will be incomplete. There are entire subject areas absent from review journals.
In terms of self-published books, there are good reasons why libraries traditionally don't purchase these books. Generally they're not very good, hence the reason they are self-published. Libraries have a limited amount of money to spend and will spend it on the books that are popular and in demand. Historically, self-published books haven't generated demand as there's no publisher creating that demand through marketing. Although books of quality have been self-published there is no way to find out which ones are worth purchasing without reading them, and libraries simply shouldn't be gambling with their budgets.
But the publishing industry is changing, and libraries need to change with it. Some books are gaining popularity through online resources without traditional marketing or reviews. A Thingology article illustrates a perfect example of this. Getting Real, a book about building web applications, was originally made available in pdf, then released in print through Lulu.com. It is a big seller, but according to WorldCat only 3 libraries own it. If it's popular, libraries should have it, but how are we to know it even exists without looking for it? There is a growing need to start looking at non-traditional sources to guide our collection development and luckily this is becoming easier.
But there's another hurdle: even if a library is willing to purchase a self-published book, ordering it isn't easy. Public libraries order with purchase order numbers and then pay the invoice later, which isn't the way that self-published books are sold. How fantastic would it be if we all had corporate credit cards and could order what we want from any vendor we want? Alas, it is not that easy. Even small presses are difficult to order from using a PO. Sadly, purchasing policy is determined by the cities we work for, so the needed changes have to happen at a higher level than the library. We just need to convince our governing bodies that it's in their interest to evolve. Good luck to us with that!