In a very narrow category, of course. But still number one.
My book “The $0.99 Ubuntu Beginner’s Guide” is, as of this writing, currently #1 in “Kindle-Shop > eBooks > Fremdsprachige eBooks > Englisch > Computer & Internet > Betriebssysteme > Linux” on the German version of Amazon.
Have I mentioned that I really like electronic self-publishing?
Of course, Ubuntu isn’t for everyone – but I do have ebooks on other topics for the discriminating reader.