I have published four books since I became a Christian, all available in paper copy (and some in Kindle) from University of Buckingham Press. These were published on the basis of an agreement (suggested by me) that - by foregoing any money - I would be able to retain copyright and publish free, online, e-text versions of the books.
My intention was that this would increase the availability and impact of the books - beyond what would have been possible (or plausible) for a small publisher.
2011 - Thought Prison - the fundamental nature of political correctness
http://thoughtprison-pc.blogspot.co.uk
- 47,000 Pageviews
2012 - Not even trying - the corruption of real science
http://corruption-of-science.blogspot.co.uk
- 18,000 Pageviews
2014 - Addicted to Distraction - psychological consequences of the mass media
http://addictedtodistraction.blogspot.co.uk
- 26,000 Pageviews
2016 - The Genius Famine - why we need geniuses, why they're dying off and why we must rescue them. (co-authored with Ed Dutton)
http://geniusfamine.blogspot.co.uk
- 5,500 Pageviews
If Pageviews were book sales then this would be pretty impressive! - but, probably they are not equivalent. It costs nothing and takes almost no time to click onto a book link; but if someone buys a paper copy of a book, they have invested more into it and are therefore more likely actually to read it.
Indeed, to read this kind of e-book pretty-much requires that the individual reader copies, pastes, edits and prints-out a copy; otherwise - if they try to read it on-screen - they are likely to be getting only a skimmed and superficial experience of the text.
In addition to the above; I have co-authored two other books - The making of a doctor: medical education in theory and practice, with RS Downie, 1992; and The Modernization Imperative, with Peter Andras, 2003. Both of these now seem, from my Christian perspective, to be essentially wrong.
The other book was Psychiatry and the Human Condition (2000) https://www.hedweb.com/bgcharlton/psychhuman.html - which I would still regard as a very good piece of scientific work (!); despite that its view of the 'human condition' is secular, materialist and hedonic.
In general, I have always realised that my books are inferior to my essays, in terms of creative achievement. I naturally write at essay length (which is why I can blog daily quite spontaneously) - and therefore have to 'assemble' and 'manufacture' my books like a mosaic from small autonomous units; even though the books are only short.
This isn't uncommon, in my experience - it seems to me that the best non-fiction prose usually falls into essay length rather than book length. For example, GK Chesterton was a better essayist than book length author - and this especially applies to writers (such as myself) who adopt an aphoristic style. An extreme would be Ralph Waldo Emerson, whose work naturally falls into aphoristic sentences or paragraphs - and who found even the essay to be something requiring artificial construction.
Books of aphorisms are indeed more-or-less un-readable qua books - Pascal's Pensees, Traherne's Centuries of Meditations, Nietzsche's works after the monograph Birth of Tragedy...
Thus non-fiction prose is much like poetry - at its best (and beyond a certain minimum length), its intensity is inverse to its effective length. Long narrative poems are either discursive and - line by line - inferior to short lyrics; or else are lyrics embedded-in longer sections of less-poetic narrative -- and much the same, mutatis mutandis, applies to non-fiction prose that aspires to artistic effectiveness.
Which of my books is best from an 'artistic' perspective - in terms of quality of writing - is something I am not likely to judge well - but at the time of writing it seemed to be 'Not even trying' - the least popular of all my books. I felt N.E.T. was as good a piece of extended writing that I could manage, with only one significant structural fault (a clunky transition of subject matter).
But, for non-fiction prose, the subject matter and views are extremely important to enjoyability: we must be interested by the subject matter, and have some kind of sympathy with the author's views to be able to appreciate it; probably because it is a polemical form, intended to persuade - and the reader must have some basic and broad-brush willingness-to-be-persuaded, if such a book is to be a positive experience.