They really shouldn’t still be selling his books (electronic or hardcopy). Selling fictional material as non-fiction is more or less fraud. Urbane publications took his books down, others who have published his books should do the same.
I completely agree with you, but I expect the counter-argument will be that there is no way of conclusively showing that this particular book is a fraud. He just might have been at that trial (he does contradict himself about that, but even so); he might have received a letter from a prisoner; he might have worked at the Court of Appeal in 2002 and spoke with Edmund Lawson (an interview that I think would have been 'in character' for Lawson); he might have interviewed those police officers.
Don't misunderstand: I'm very sure he is a fraud and it's all made up, but there is a burden on the accuser if the matter is to be taken further and there are ways for him to wriggle out of it or muddy the waters. For instance, one could ask the Ministry of Justice whether they have any personnel record for somebody called Paul Harrison, but even if they can positively deny he was ever employed by the courts, he could say that he was there as a contract worker or in some capacity related to the police, or whatever.
Personally, I view his book along the same lines as the online book, David Shaw's The Innocent Man. It's a work of pseudo-fiction on the killings, as well as a spontaneous psychological study of the author himself, and perhaps should be read, studied and considered from those angles?
The real issue I have with Paul Harrison is that his book is just not very good. I could forgive him if it was a good read, in which case I would just put it down to a bit of enterprise and consider it amusing. Although David Shaw's online work is blatantly plagiarist and fictitious, it is at least a good read and a ripping yarn. I had the popcorn out reading Shaw, whereas with Harrison I was falling asleep. Never mind that it's made up, the real problem is that it's badly-written and boring.