Leonora Piper was a clairvoyant medium who cooperated with psychic researchers between 1889 and 1910. The investigators concurred with remarkable unanimity that her abilities were genuine. They included the philosopher William James, also Richard Hodgson, an Australian researcher who made his name by debunking the psychic claims of Helena Blavatsky, the founder of Theosophy, and who was sceptical about other mediums such as Eusapia Palladino. Another was Frank Podmore, an SPR researcher who took a sceptical position about almost everything else that his colleagues considered supernormal.
Much of Chapter 3 of Randi's Prize argues the case for Piper, and against the assertions of sceptics who in the main appear not to have read the primary research.
Two key articles, papers by Richard Hodgson on different phases of Piper's mediumship, can be downloaded here as pdf scans of the original text.
See also this excellent critique by Greg Taylor of an essay by sceptic Martin Gardner that attacks the philosopher William James for letting himself be 'bamboozled' by Piper. As Taylor points out, Gardner put forward arguments that he could not legitimately have made if he had read the primary sources.