I'm still working on the fourth Zeena book. I'm not as far along with it as I'd hoped to be by this time, partially due to the fact I was away for almost a week towards the end of February, and didn't really do any work on it while I was away, but also partially because progress on it is slow in general. If you didn't notice I was away, that's OK... You weren't meant to. I'd set up some blog posts before I left, so while I was gone posts continued to appear on my blog on the days I usually would have posted, which meant I only had to catch up with comments when I got back. Well, those, and what felt like a mountain of e-mails.
If you're working on a book of your own right now - especially if you're currently in the re-writing or editing phases of your work - you should check out these 6 grammar mistakes even smart people make, so you can try to avoid making them yourself.
Also, which version of English do you think you should use in your book? Personally, I think you should use the version that's most familiar to you. In other words, if you're British, use British English. It's British English I use as a rule, though the odd bit of American English or Canadian English may slip in there from time to time, since I've spent so much of my life talking to people from all three places that the lines are slightly blurred for me. There are parts where I know for certain x is British English but y would be American English (where x is the word or phrase used in the UK, and y is the alternative used in the US that has the same meaning). But sometimes I'm not even certain which is which, and where the difference is a regional one, or based entirely on the fact there's often more than one way to say something.
If you want more writing tips, you could always check out this creative writing course. It's free, and seems to be worth checking out; I haven't done it myself yet, but others seem pleased with it, and I plan to check it out in the near future.
Are you a children's author too? If so, are some of your books available as paperbacks? If they are, would you be willing to donate some of your books to a charity for special needs children? If the answer to all three of those questions is yes, you should check out the article I just linked to, and send some books to Angie. Or, if you can't send your books to Angie for any reason, you could see if there are similar organizations near where you live, which you can send to instead. After all, encouraging any child to read is a good thing, don't you think?
Speaking of reading... Here's a simple, yet important, writing tip for you: reading is not optional if you're a writer. This is why I spend most of the time I don't use for writing devouring books by other authors, and why a large number of the books I read are aimed at children (though I do read books in quite a variety of genres, and for any age group).