You might explore Howells' correspondence with other writers and men and
women of letters.   What did he say about Twain to Henry James, for
instance?  Or to Thomas Wentworth Higginson?  Or to writers of the
previous generation (like Holmes) who lingered on well into the 1880s?

Howells' range of acquaintance was SO broad and the late-Victorian
intellectual world SO densely interwoven that interesting remarks by
Howells about Twain may be scattered among a dozen correspondences.

Mark Coburn