I know you are new, to both AW and the publishing world, so I hope you understand that the following is meant to be helpful advice. We've all been green at this, trust me, my ignorance about the publishing world when I got started was kind of embarrassing. It's a steep learning curve, and ever since I became more educated on the subject, I've tried to do the same with others. So please take all my advice in the spirit it was given . . . uh . . . which would be "helpful". I hope
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My book is currently 38,468 words.
What genre is your book? Unless this is a book for kids (and I do mean kids, 8 - 12 yearolds, not Young Adult. Young Adult - you know, like TWILIGHT, is longer usually 50K - 70K (though TWILIGHT is around 115K) ) - this word count is too short for any full length novel. There are always exceptions to the rule, but truly, this is a very short word count and will get you automatically rejected by publishers. HOWEVER, if your work IS for kids, or even Young Adult, you might have a bit more leeway with a short word count. In this case, it will depend more on the publisher's requirements.
Before I start submitting it to publishing houses, my mom has sent it to a professional editor.
In the UK it is common for authors to use book editing services, it isn't as common in North America. Typically it is considered best for authors to be able to edit their own work. However sometimes an author lacks anyone else to read their work for them, and going with an editing service can make sense. You might want to see if anyone here would be willing to read your work and give you their thoughts, though. There are some excellent fellow authors here who are very good at helping each other, and they do it for free.
I'm nervous that they're going to take out a lot of the text since I use a lot of imagery (but I've read books with a lot of imagery). Hopefully, that won't happen, but how do professional editors normally edit books?
This really isn't how a book gets edited. Good editors might make grammar and spelling corrections automatically, but when it comes to style and content, they ought to make notes in the margin, offer their advice, and then ask you to solve the problem. So if an editor doesn't like a descriptive passage she'll write something like, "I don't think all this description works right here, is there any way to cut it down?" and then you as an author decide if you want to. Sometimes you agree with the editor, a kind of hand smacking the forehead, "D'oh why didn't I think of that" moment. Sometimes you don't agree, though, and then you keep it as is.
I am pretty sure the ones in the big houses (i.e. Random House) change a lot of the story.
You are wrong. Editors at the big houses want books that are a polished as possible. It's a really nasty myth that editors at big houses buy an author's work and then change everything. Editors are book lovers and buy what they love. Why would they then want to change what they love?
Also, if the editor takes over too much, no one will really know what is my true style of writing.
This is true. And a bad editor is one that changes your work at all, let alone beyond recognition. The good news is that you don't have to accept any of the changes. If an editor ruins your work, get a refund and keep your book as you originally wrote it. You aren't obligated to an editor, even one you pay.
Basically, how do professional editors edit books? Is it a lot or a little?
It depends. But the thing that is most important is that no good editor will change something an author doesn't want changed. And no good author will take what an editor says with blind faith.
Good luck with everything, and welcome to AW!