Supply and demand mate. The guy’s won three/four world titles (depending on how you look at it) and is Britain’s most popular boxer… that’s an opportune time to write an autobiography at a point when a lot of people recognise him. And he’ll prob do another (hopefully with more world titles and mega fights to talk about) and we’ll all buy that as well.
Also bear in mind the market mate… he’s got a relatively small market to aim for compared to, say, a footballer so fair play to him for releasing one so early. It’s good business sense.
As someone else indicated… name of new autobiography: “How I tore Witter a new ar*ehole”