I think the best of his novels is The Man Who Was Thursday (and I'm amazed to find a dramatisation of it: that's downloaded for later, and thank you for pointing me at it!!). The Father Brown short stories are great fun, and about the only thing he's remembered for. To get an idea of how he wrote non-fiction, you could try Orthodoxy, of which I am very fond. The essays and the poems...as you say, too many: just dive in and see what you find.