Suppose you’ve written a book which has received nothing but rejections, but you don’t understand how you could improve it. One way to get specific advice is to pay for an editorial assessment of the book which should point you in the right direction.
Editorial agencies are not without their problems: there are plenty which offer very expensive bad advice; and there are a few which are out-and-out scams (Edit Ink, anyone?). Some, though, provide a useful service: Sharon Maas, who had three novels published by HarperCollins, doubts that her first book would have been published at all had she not first worked with an editorial agency to improve it, and I’ve heard good things about others, too.
Writing a good book and using a good editorial agency to improve it won’t guarantee immediate publication, though, as Sally Zigmond discovered; and even the good agencies won’t always get it right—if you write in a genre they aren’t familiar with, or in a style they don’t particularly like (regardless of its literary merit), then you’re not going to get the best advice. And using the wrong agency for your particular book, even if it’s a good one, can be an expensive mistake to make: those reports can cost hundreds, if not thousands, of pounds.
If you’re determined to use an editorial agency to improve your book, how should you proceed? You need to find a few reputable agencies: either through personal recommendation, or through your own research. They must be run and staffed by people who are qualified to do the work: either by having appropriate editorial experience, or by being published writers themselves. They must be able to demonstrate that their reports have been effective: most will provide a list of clients and testimonials. And when you check, you’ll find those testimonials corroborated elsewhere, by people who are not in the agencies’ employ, and who have had a good degree of publishing success as a result of the advice that they received.