Non-fiction is much easier to sell than fiction. And now I realise why.
First, let me tell you how I realised that this was even the case. In August I published my book on Twitter - Tweet Right - The Sensible Person's Guide to Twitter. And in November I re-published Mondays are Red, which was my debut novel back in 2002. I published both books as ebooks only.
Now, Mondays are Red should have had an advantage because it has been published before and has a raft of lovely reviews from newspapers as well as readers; also, it's been out of print for a couple of years and people are still asking for it. But it's selling about a quarter of the number that Tweet Right sells on a weekly basis. (Which is not a vast number, let me tell you, but it's very respectable.)

And this is despite the fact that I did more to push Mondays. A blog tour, for example, which I didn't do for Tweet Right. And TR is more expensive. And shorter. It is, by word count, much less good value. With Mondays are Red, I pleaded with my blog readers and employed blatant emotional blackmail. I never did that with Tweet Right.
However, none of that really worked. (I'm actually a bit relieved - I don't like pleading or blackmail! And btw, let me be clear: I do NOT expect people to buy out of duty.) So, TR continues to outsell Mondays by about four times.
And it seems to me the reason is obvious.
When you try to persuade a reader to buy your novel, you're trying to persuade them to want this one more than thousands - hundreds of thousands - of others. Even if yours is the genre they like to buy, you're still competing in a crowded, often poorly differentiated market. It's easy to be invisible. (Especially since there are some things I won't do to get myself or my book seen.)
But if they are looking for a book about something - Twitter, or my next topic, writing synopses (Write a Great Synopsis - An Expert Guide, coming in January!) - there are very few books that I'm competing against. Very few indeed. It's easier to be seen. Also, it's relatively easy to find the audience, because you know where they hang out. But readers of novels are everywhere, everywhere, I tell you. And they are slippery. God, they are.
So, it's obvious when you think about it, isn't it? It's much more a numbers game than we'd like to think.
In view of this, I will not bother to plead with you to buy Mondays are Red. Honestly. Don't. There are hundreds of thousands of other novels you might like almost as much. But, on the other hand, there's only one at the other end of this link. :)
EDITED TO ADD: I have a suggestion: if any published* UK authors with YA titles which are also in ebook format for Kindle would like to get in touch, I'll do a blog post (on my blog) after Christmas which will list them, with links. SO, if your book is YA, published and in ebook format, email me, in this order: title, author, publisher, 25 words to describe including genre, and Amazon link (UK or US, just one). By Christmas Eve. n@nicolamorgan.co.uk
(* I'm really sorry but I have to offer this only for authors who have had novels published by a trade publisher. This is purely so that I don't end up having to put eleventy million books in a blog post when I could be eating mince pies.)