I was given about half a dozen of Rankin's books several years agao and dutifully read them all, though I can't now recall any of the titles. I have to say I found them a bit wearying after a while. Some of the humour is funny, but it goes on and on, like a child who has found a joke book and has to read - out loud - every single one.
And he shares with early Pratchett the deliberate twisting of plot and character just so that he can lead up to a joke at the end of it. But whereas Pratchett evolved his style to write fewer jokes but more genuinely witty lines, the Rankins seemed to stay at the same level - almost as if he had a contractual obligation to have a punchline on every page. Subtle it is not.
J