Howells included most of his reviews of Twain's books in My Mark Twain, but
you might check Tenney to see if he lists any that were left out. In my own
copy of MMT Howells scribbled that it took him two weeks to write the book,
a claim I would discount, but the text does evoke a faint odor of paste from
Howells' cut-and-paste approach.

Of course, the MT-Howells Letters are a rich source of Howells' private
commentary on Twain that complements his public statements.

And Howells presided over the Nov 30, 1910 American Acad. of Arts & Letters
memorial where he made some brief introductory remarks that are surely
clever Howellsian rhetoric, but still telling:

"...sanguine, sorrowful; despairing, exulting; loving, hating; blessing,
cursing; mocking, mourning; laughing, lamenting; he was a congeries of
contradictions, as each of us is; but contradictions confessed, explicit,
positive; and I wish we might show him frankly as he always showed himself.
We may confess that he had faults, while we deny that he tried to make them
pass for merits. He disowned his errors by owning them; in the very defects
of his qualities he triumphed, and he could make us glad with him at his
escape from them..."

Kevin Mac Donnell
Austin TX