My first published novel, Hamelin's Child, was agency-edited. The two follow-on books, I was reasonably confident about - although I do have a supremely-talented beta reader who can spot a typo at a thousand paces and is not shy about telling me the bits that don't work. I am eternally grateful for his support.
But when I got to my spin-off book, Rat's Tale, I was no longer so sure about what I was doing. Rat's Tale was shorter than my other books - more of a novella at under 50,000 words. And I wanted to do something different with the structure that I hadn't done before. I wanted a fresh pair of eyes and I was happy to pay for it - it's no different to buying-in design services for a book cover, or formatting (although I do my own ebook and print formatting). By this time, I was socialising widely on facebook and John Hudspith's name kept cropping up, so I thought I'd try him out....
So how does engaging the services of an editor work? With John, you get a free sample edit, so you know exactly what you will get for your money, should you choose to accept his quote. Your quote will be based on the sample you submitted, and you will be able to see for yourself how much/little support you need. He'll fix typos, suggest alternative words and correct punctuation and spelling. He'll also point out style issues, where perhaps sentences aren't flowing as well as they could, or where scenes need expanding or rewriting. He's also exceptionally good at cutting out fluff - those extraneous words that creep in without you noticing and threaten to smother your story. And he'll suggest rewrites where necessary (editing of rewritten scenes is included in the price - as are blurbs and synopses).
Most fiction editors offer similar services. Some are fixed-fee, some charge according to the sample. Many editors will ask for all/part payment up-front. John works in chunks of approx 10,000 words - so you can budget as you go along.
Whoever you choose - if you choose to employ an editor - it's important to have a rapport with him/her. You need to feel comfortable and able to discuss and query/argue with suggestions. Always make sure you know what you are getting for your money. Ask for references if necessary and make sure you are happy with the quality of the sample edit.
So what happens to my books? I have lots of fluff. I could stuff pillows with my fluff. Chapters coming back from Johnny have been positively shaved. I have typos - everybody has typos - and sometimes I overuse the same words or I get lazy with my research. I also have a habit of building up the tension and then losing it because I've not gone quite far enough to reach the point of no-return. I can't usually see it myself, but Johnny can. Oh, and he puts even more swear words in than I started with (and I'm not exactly mean with them), and sex too - so I'm blaming him entirely for corrupting the moral fabric of society ...
Lists of other editing services can be found here. Caveat emptor! Always ask for references or testimonials from satisfied customers before parting with your money.