Letter from Peirce to Harris (1868.1.24)
(engl.)
at Peirce Edition Project
[www.iupui.edu/~peirce/writings/v2/w2/w2_13/v2_13.htm]

Letter from Peirce to Harris (1868.3.16)
(engl.)
at Peirce Edition Project
[www.iupui.edu/~peirce/writings/v2/w2/w2_15/v2_15.htm]

Letter from Peirce to Harris (1868.4.9)
(engl.)
at Peirce Edition Project
[www.iupui.edu/~peirce/writings/v2/w2/w2_17/v2_17.htm]

Letter from Peirce to Harris (1868.11.30)
(engl.)
at Peirce Edition Project
[www.iupui.edu/~peirce/writings/v2/w2/w2_20/v2_20.htm]

Mr. Peirce and the Realists (1871.12.14)
(engl.)
at Peirce Edition Project
[www.iupui.edu/~peirce/writings/v2/w2/w2_50/v2_50.htm]

The Charles S. Peirce-Simon Newcomb Correspondence (letters, The correspondence is presented with an extensive biographical commentary by Carolyn Eisele)
(engl.)
at Google Ricerca Libri
[books.google.com/books?id=WlkLAAAAIAAJ&pg=PA409&dq=The+Charles+S.+Peirce+-+Simon+Newcomb+Correspondence&sig=6vWck5WsbBv7gYi15HMqqG0rP9E#PPA409,M1]

The Peirce-Langley Correspondence and Peirce’s Manuscript on Hume and the Laws of Nature
(engl. ,ed.: a c. P.P.Wiener, The American Philosophical Society, Philadelphia 1947)
at Google Ricerca Libri
[books.google.com/books?id=fpWH6doabbYC&pg=PA201-IA1&dq=Peirce,+Charles+Sanders&lr=&as_brr=1&sig=7IOBJzOurdJ9U3skAO5dbB8J4nE#PPA201-IA1,M1]

The Solar Eclipse of 7 August 1869 (1869.8.20)
(engl.)
at Peirce Edition Project
[www.iupui.edu/~peirce/writings/v2/w2/w2_27/v2_27.htm]