There is a lengthy entry on Arthur Conan Doyle in Alan Gribben's
_Mark Twain's Library: A Reconstruction_ in which Gribben provides
a number of references to Doyle's possible influence. Doyle did
admire Twain's _King Leopold's Soliloquy_ and sent him a copy of
his own _The Crime of the Congo_ in 1909. However, to describe
their relationship as an old friendship is probably an exaggeration.