If you don’t plan to publish many books then you can use self publishing services such as Kindle or Lulu, which is a service that sits in front of a printing company called Lightning Source. Lulu offers basic formatting, ready-made cover designs and ISBN number allocation, as well as an online book shop.
Some services charge a set up cost, others offer free set up but hide inflated printing and distribution costs, recouping their set up costs from book sales.
Self publishing means that anyone can publish a book with no critical assessment, but in reality their book doesn’t reach a significant readership through book shops, so themselves critical review process takes place at the point of sale rather than at the point of writing, editing or publishing.
If you are considering self publishing, it is important to at least have someone else proof read and edit the book, as it is very easy to overlook mistakes when you’ve been staring at text on a computer screen for hours.
Self publishing removes all barriers to entry, apart from money, so there are no quality controls or checks on books entering the market. This can leave readers with the perception that all self published books are of poor quality.
Having a publisher means that your book has been evaluated and edited to be of publishable quality, and that gives your book credibility.