The Private Papers of Henry Ryecroft (1903), by George Gissing. Apparently one of H.P. Lovecraft’s favorite books, and one on which he based his personal philosophy of life. Sonia stated that she received a copy of it early in their courtship, with instructions to read it to better understand him.
Curiously the book isn’t listed in Lovecraft’s Library. My guess on that would be that S.T. Joshi had to weigh Sonia’s solo revelation about the book, against Lovecraft’s apparent utter silence on Gissing in all his other walks of life and voluminous correspondence. And the similar silence of his bibliophile friends on the book. But if you’d like to take a look, the book is available here: Dutton U.S. first edition online digital facsimile and as .mobi for Kindle.