That is a shame. Mark of a great writer though, that 12 books seems so little.
He wrote some other stuff too, some of which is worth seeking out. Flashman's father appears as a secondary character in 'Black Ajax', the story of the first world boxing championship fight.
And his war memoirs - 'Quartered safe out here' is brilliant, and the McAuslan stories are hilarious. 'Mr American' is another period piece, but a serious story, unlike the Flashman books.
Some of the other stuff should be approached with more care. 'Pyrates', for instance, piles on the silliness and slapstick until it's really not as funny as GMF may have thought. And 'The Light's on at Signpost' is pretty much unreadably bad.