Penton and Willis show up in footnotes. I don't think he lets his status as a Witness get in the way of scholarly standards. However, most of this is taken from primary sources. Very little of it, almost none of it, is taken from secondary sources. Looking through an early draft of their chapter three as found on their private blog, I find these footnotes as samples of their sources:
[1] J. L. Russell & Son to G. Storrs, Letters Received, Bible Examiner, April 1874, page 130. We do not know which of the Russells sent the letter. One researcher claims to have proof of contact between the Russells and Storrs as early as 1869. The proof is never forthcoming. We could not find it. The claim does not seem to fit the known facts. Consider this footnote a challenge to “put up or shut up.”
[2] G. Storrs: Visit to Pittsburgh, Pa., Bible Examiner, June 1874, page 259.
[3] Letter from J. L. Russell to Storrs and note appended by Storrs, Bible Examiner, November 1874, page 66.
[4] Parcels Sent to May 25, Bible Examiner, June 1874, page 288.
[5] Letters Received and Parcels Sent, Bible Examiner, November 1874, page 64.
[6] Margaret Russell Land’s testimony found in Souvenir Notes from the Watch Tower Bible and Tract Society’s Convention, 1907.
[7] George Stetson to his daughter Kate, March 23, 1875. The letter is found in the Thew-Stetson Archive at the State University of New York at Plattsburgh.
[8] Storrs’ attendance: Charles Algernon Downs: History of Lebanon, New Hampshire, 1761-1887, Rumford Printing Co, Rumford, New Hampshire, 1908, page 430. Downs is our sole source for this. We would be happier with supporting documentation. School’s purpose: Convention on the Subject of a Seminary, The Panoplist and Missionary Magazine, December 1812, page 329.
[9] Emails from Jane Fielder, the school’s archivist, dated February 21 and 22, 2011. Ms Fielder searched the school’s records with no result but observed that the records are incomplete for those years.
[10] O. W. Muelder: Theodore Dwight Weld and the American Anti-Slavery Society, Jefferson, North Carolina, 2011, page 89. Storrs ministry in this period is well documented, presenting this record: Admitted on trial to the New England ME Conference 1825; Ordained deacon by Bishop Hedding at Lisbon, June 10 1827 and elder by the same at Portsmouth, June 15 1829; Appointments Landaff, 1825; Sandwich, 1826-7; Gilmanton and Northfield, 1828-9; Great Falls, 1830 and 1832; Portsmouth, 1830-1; Concord, 1833-4; Henniker and Deering supernumerary 1835; left the Methodists 1840; Without charge, Montpelier Vermont, 1841; Supplied Albany, New York, 1841-2. – See N. F. Carter: The Native Ministry of New Hampshire, Concord, 1906, page 428.
There is, as I said, almost no dependence on secondary sources. This is fresh, original research. I noted on another site that Penton said they were going where no one had previously gone. This is true. This is no rehash of what others have written, though they've obviously read it all.