The Mr.Men Conspiracy

Books 1-43 of the Mr.Men and 1-30 of the Little Misses have 'by Roger Hargreaves' on the front cover.

Four of those Mr.Men books were published in 1990. Nine Little Misses around the same time. Roger Hargreaves died in 1988. 

It seems Adam Hargreaves, son of Roger, took over writing and illustrating the books some time after 2000. While his books still have 'Roger Hargreaves' on the front, they are clearly credited on the inside.

So... who wrote and illustrated those '90s Mr.Men books that are passed off as the work of Roger Hargreaves?