Raymond R. Borst, one of the world's leading researchers on Henry David Thoreau, died Friday, April 27. He was 91.

Borst wrote three books on Thoreau and amassed one of the largest private collections of original Thoreau works in the world. The Thoreau Log, published in 1992, traces the day-to-day activities of the great transcendentalist, from cradle to grave, in nearly 700 pages of minute detail. Borst's first book, a bibliography of Thoreau, is so exhaustively complete that virtually every Thoreau offering in the catalogs of rare book dealers concludes with Borst's name and a reference to his bibliography.

In 1996, Borst donated his world-class Thoreau library of more than 500 books and other printed items to the University of Rochester, from which he graduated in 1933. Among the strengths of the collection are 16 early editions of Walden, Thoreau's most famous work.

Borst purchased his first copy of Walden at a hotel bookstore more than half a century ago for his wife, Anne, who loved nature and Thoreau's philosophy toward it. That first copy became the seed of the enormous collection. Raymond and Anne Borst continued to gather Thoreau manuscripts, even duplicates, expanding the collection and prompting the construction of a wing on their farmhouse in Auburn, N.Y., to accommodate their growing library. Anne Borst died in 1977 at the age of 66, by which time the collection had become one of the largest and most complete assemblages of rare Thoreau writings in the world.

A year later, the University of Pittsburgh asked Borst to compile a comprehensive bibliography of Thoreau; an accounting of everything ever published by the author, every appearance of every book, article or speech.

"For most of us, it is folly to contemplate bibliographical work at all," says Peter Dzwonkoski, curator of literary collections at the University's Rush Rhees library. "It requires a certain kind of mind, a gift for exacting detail and infinite patience and determination. Ray was a successful businessman who produced the kind of work usually done only by university professors and scholars."

After the acclaim of his 1982 book, Borst went on to write Henry David Thoreau: A Reference Guide, and in 1992, The Thoreau Log. Dzwonkoski calls this last work "astonishing." Borst poured over thousands of letters, journal entries, newspapers, public notices and even library lending records to reconstruct what Thoreau was doing on nearly every day of his life.

Borst transferred the entire collection to the Department of Rare Books, Special Collections and Preservation, with the help of a substantial grant from the Viburnum Foundation. Now named the Raymond R. Borst Collection of Henry David Thoreau, the compilation anchors the library's already extensive holdings in 19th-century American literature.

Three weeks before his death, Borst discovered a small Thoreau chapbook mislaid in his house. He delivered it to the University's rare books curator with the words, "I wasn't holding out on you. This one almost got away."

Borst was born in Auburn, attended Hamilton College and graduated from the University in 1933. When he purchased his house in Auburn, it, a bit like Thoreau's, had no plumbing and scant electrical wiring. Borst leased the land to farmers while he took over a truck- and farm-equipment business from his father, renaming it Borst Brothers. He was a vigorous member of the community, co-creating the Sennet Fire Department, serving as director of the Auburn Chamber of Commerce, president of the Cayuga Museum of History and Art, and president of the Schweinfurth Memorial Art Center where a room remains dedicated to Anne. He also served as president of the Auburn Rotary Club, member of the Foundation Board of Seymour Library, and member of the board of the Thoreau Society. A few years after giving his wife the copy of Walden, Borst bought a small cabin in the Adirondack Mountains along a river. It was there that he spent most of his summers, and where he wrote and edited much of his books. Never one to stop learning, Borst bought a computer in his 80s and taught himself to use the Internet in the same house in which he had been without an electrical outlet 60 years earlier.

Borst died of a stroke at Strong Memorial Hospital in Rochester, N.Y. He is survived by his daughter and son-in-law Cynthia and Richard Sherwood of Rochester, daughter Joan Borst of Sheridan, Wyo., five grandchildren, Elaine, Elizabeth and Jonathan Sherwood of Rochester; Anne and Jennifer Sherwood of Los Angeles, Calif.; and three great-grandchildren, Maximilian and Grace Plantamenta, and Blythe Jakubowski of Rochester.