Once you've published your book, you face the problem of how to get it out there to the customer. We all know the stereotypical image of the author with boxes of their own books under the bed gathering dust. Distributing your book yourself takes a considerable amount of time and effort (time which I would prefer to spend illustrating) and you also need a lot of nerve. One of my friends recently bought 300 copies of her own book. She's retired so has the time to do the footwork around bookshops and libraries, and she has had some success. It seems that my grandma's mantra of "if you don't ask, you don't get" is really true, but much of the time I don't have the nerve to ask.
The problem of distribution is largely solved by using a 'self-publishing' company but unless you have a magic porridge pot of money, these vanity presses are best avoided.
When it comes to true self-publishing and print on demand, Lulu, CafePress and CreateSpace are making it more and more viable for authors to publish their own books. It is worth looking at all of these, comparing costs and considering how they fit in with your needs. If all you want is to produce a few copies of your book for friends, family or your community group, then they may well be your best bet. Both Lulu and CreateSpace now offer a free ISBN and selling via Amazon.com, great if you are US based.
As a UK writer and illustrator though, it was important to me that my books were readily available in the UK and through the UK Amazon. By chance, I learnt that many POD books in the UK are produced by Lighting Source, who also have branches in the States and in Europe. I contacted them and they confirmed that they would be willing to work with me, providing I could create print-ready files to their very specific requirements (what a learning curve that was!). Lightning Source has set-up fees which can make it more expensive if you only want a few copies of your book, but if you are aiming high and UK sales are important, then it comes into its own, printing costs are very low and the quality high. Your books will be sold not just through Amazon, but Ingrams, Barnes and Noble and many more. I'm often surprised just where my books appear! You never need to worry about handling stock or shipping charges, will receive royalties on a monthly basis, and best of all it frees you up to concentrate on promoting your book and on creating more.