I've been reading books by indie authors this year and reviewing the best ones on GoodReads and, more lately, on this blog. The theory is that other indie authors will read and review my books in turn, so common sense dictates that if I don't like a book, I shouldn't review it. 
Here are some of the great books by indie authors I've read recently: 
Hvaldi by K.J. SimpsonOil and Water by Lara Ann DominickAngie Baby by Terry KerrAngel of Mercy by Melina DrugaBucky and the Lukefahr Ladies by Shirley Gilmore
These (and many others) are of the same high quality as books published by traditional publishing houses. They tell imaginative and compelling stories with convincing characters and dialogue. The challenge for authors like these is to bring their books to the attention of appreciative readers.
Although all my reviews are honest, I haven't reviewed all the self-published books I've read. Sometimes it only takes a few pages to establish that a particular book isn't for me -- in which case my review probably wouldn't help anyone -- but more often than not, if I finish a book and don't review it, it'll be because of the technical quality of the writing.

As an indie author, you put a great deal of time and enormous energy into writing, publishing and marketing. When your book's written, there's still a lot of work to do and because you're not working with a publisher, you have to do it yourself or pay someone to do it for you. This can be expensive because it's skilled and labour-intensive work. Although we all want our books to be as good as they can be, there's no guarantee we'll ever recoup the cost. 

There's no getting away from the fact that some great storytellers who self-publish aren't great at grammar, spelling or punctuation. They probably don't realise this and wouldn't be grateful to be told. These aren't the indie authors I'm addressing here. If you know how to put together a sentence already, you want to produce books of professional quality and you care about the details, I'm talking to you. 

As your publication date draws near, time speeds up. You might go back to the text to re-read it, but if you do, you'll probably tinker and add new mistakes. It's hard to proofread your own work at the best of times, because you know what you meant to say, but when you're working under the pressure of a deadline, it's next to impossible to focus on the details. 

Perhaps you'll ask a friend or relative to read it, just for reassurance, while you get on with one of the hundred other things you need to get done before your deadline. Unfortunately, proofreading is a specialised skill and helpful friends might catch a few typos, but they're unlikely to catch them all. 

Even reputable presses sometimes publish books with typos, so it's not surprising that indie publications often include a few more. Usually it's obvious what was intended: a homophone has slipped in (e.g. yolk instead of yoke), an apostrophe is missing or inserted where it shouldn't be (e.g. it's for its, or vice versa), or the subject of a sentence has been changed (e.g. he becomes they) without the verb being changed to match. These aren't the mistakes of authors who can't write: they're typos arising from momentary and understandable lapses of concentration.

When you work with a publishing house, you have to conform to their house style. There are hundreds of possible variations in spelling and punctuation and the house style ensures consistency. While you're writing, you make decisions as you go along between:

judgement or judgmentgipsy or gypsyvictimise or victimizesingle or double inverted commas for speecha final apostrophe or 's for possessives ending in s (and so on) 

None of those decisions is wrong, but it's very hard to be consistent without an eagle-eyed editor checking them against a style-sheet. 

Perhaps this is the moment for me to come out as a pedant. Although (perhaps because) I don't have a visual memory, I do notice details of language. If you say a character has red hair after describing them as blonde, there's a good chance that won't bother me because I don't have a mental picture of them. But if you miss out an essential comma or use British spellings in a letter from a speaker of American English, that will grate. If you start out using single inverted commas and switch to double halfway through, I'll notice. If you use different spellings of the same word, I'll spot it. If you accidentally put 20th century slang into the mouths of your 19th century characters, I'll grind my teeth down to the gums, I really will. Please know that this doesn't make me happy.

I'm 100% confident in my writing and proof-reading abilities. I'm a professor of English and I spent 20+ years studying and publishing on the history of English. I've corrected thousands of students' essays, acted as a consultant for eminent dictionary publishers and reviewed other academics' work for a wide range of publishing presses. This stuff is easy for me. 

I'm less comfortable, as an indie author, with marketing and publicity, but maybe that's easy for you.

So, here's the deal:  

I'll read your self-published book before publication to check you've caught all the typos. In return, you'll buy and review one of my books and spend a little of your time helping me to promote it. 

What I'm offering is a final polish, not a full proofreading or copy-editing service. You'll need to believe your book's ready for publication already. If there are too many errors, it won't be worth my while to go through it, and my decision not to go ahead could be profoundly unsettling with your publication date hurtling towards you. I'm happy working with all varieties of English, but you need to be writing with native-speaker fluency in whichever variety it is, so that if I query a usage, you'll know whether what's on the page is correct.

In return, you'll buy, read and review one of my books and promote it to your social media networks. I want honest reviews, but it wouldn't make sense to put my time in for negative ones, so you'll need to read my book first to see if you like it. Meanwhile, I'll read a sample of yours to see if I can do the work needed in the time available. That way we'll both be happy with the bargain we're making.

If you're interested, please send me your email address by direct message on Twitter, Facebook or Instagram and I'll let you have the full T&Cs. Looking forward to hearing from you!