I'd write the former, though both are correct, if only because it's one letter shorter (laziness!)

I'd also *say* the former, again because it's easier, also it can be understood as "Keats poems" where "Keats" acts as an adjective. Though I'd probably be even more likely just to say "a volume of Keats" since he didn't really write much significant that wasn't poems.

I'm pretty sure I *do* have a volume of works by John Keats, by the way. (Can't be bothered to go downstairs to check, esp as study/bookcases *still* not sorted since moved house so not always easy to find things.)