First, I am glad that you liked it. Secondly, you are under no obligation whatever to do anything more, but if you want to show some appreciation, consider sampling one of my other books. The Penal Colony is a loss leader, made free in order to bring my fiction to wider attention. It is far from being my best book. The others available are these:
The Stone Arrow (approx. 71,000 words). My first published novel. It won a prize, putting me in the company of people like Shiva Naipaul and Kazuo Ishiguro, which astonished me (in case there had been some mistake, I hurried to the bank on my bicycle to deposit the cheque for £500 – nearly as much as the entire advance I received from the London publishers). It's set in Sussex in the latter part of the Stone Age, when the native hunter-gatherers are being displaced by immigrants from the near Continent, agriculturalists practising slash-and-burn. The form is that of a thriller, with lots of derring-do. It's very ingenious in parts, though I say so myself.
The Flint Lord (72,000 words) seemed like a good idea at the time, a sequel to the above. One reviewer described it as "dour", and I agreed with that. However, on a later reading it seemed quite inventive and entertaining in its way, and the violence peculiarly innocent. The Flint Lord himself is a proto-Nazi, and to get through the business of writing about him I listened to Wagner via headphones. The book was generally well received, and if you like its predecessor you'll probably like this too.
The Earth Goddess (74,000 words). This follows on from The Flint Lord, and completes a trilogy I named The Pagans. It is much less violent than the other two, and the most interesting of the three. The late Neolithic and early Bronze Age threw up the first priestly class in Europe and their achievements (e.g. Stonehenge) were remarkable. The hero is a bit of a pill, and I ended the story too abruptly (by then being deeply disillusioned with my publishers and hence the whole idea of authorship), but the rest of it I still like.
Refuge (83,000 words). A post-apocalyptic thriller, my least favourite child. Publishers' readers complained that the villain, in an earlier draft, was insufficiently unpleasant, so I obliged them and rather wish I hadn't. Nonetheless, some people really like the grittiness of this book, especially in the action sequences.
The Tide Mill (125,000 words). Set in the 13th century, this describes two conflicts: one private and very personal, an illicit love affair, and the other grand and public, between the Church of Rome and the King of England. We see at first hand how a medieval manor worked and learn quite a bit about medieval engineering, but above all this is a gentle and rather touching coming-of-age story.
The Drowning (125,000 words). My latest and most overtly literary novel. It will appeal especially to British readers over the age of 50. I consider this to be some of the finest work of which I am presently capable.
Follow the links in the "Find My Books At ..." tab at the side and download the sample(s) of your choice from the website of your choice, free of charge of course. Or you can simply browse the text at Smashwords.
Update, 18 October 2012: The Penal Colony is now free again at Amazon and elsewhere, so I have re-dated this post. Since it first appeared, I have also released Darling Brenda (see panel at side).