Money is a great motivator. With only twenty years police service his pension will likely to be reduced by 50%.
Well, to be fair to him, he talks throughout the book about how much he spent on his investigation, in terms of time away from his security business and novel writing, petrol costs in driving up and down the country to interview people, and I assume paying his researcher who did a lot of the background work. And the book is self-published I think so there would have been costs in that and no advance. For him to make money, he would have to have been right, found a body and made money from book sales and media appearances or increased business. He hasn't done that.
From how it's set up I think he did want to have a shot at finding SJL's remains and he presented his work to the police before he published the book, he says that he published after they did nothing.
I believe him that he wanted to find SJL, but I feel uncomfortable about some of the way he presents things in the book (he sets up his interview with the relief landlord in a way that makes the guy look weird, odd, guilty--it's not neutral. He presents the interview with that man's ex girlfriend as also making them look guilty when there are a million reasons why people split up and are not on friendly terms afterwards and he has no idea about why). He sets it up like a novel, in a way, dropping hints that the reader can pick up on and come to the conclusion about whodunnit themselves. SJL's ex boyfriend AL is also presented as "weird" when again, there are good reasons for why he might get exasperated and walk out.
I also feel a bit uncomfortable about him doorstepping people who really don't want to talk to him. I can see his arguments for doing that, but it is invasive. The interviewees don't know him and it's a sensitive and upsetting case. Noone wants to be a The Sun or Daily Mail victim.
Maybe he passionately believes the person he all-but-named is guilty but he has put him at risk for sure.