AMHERST — Friends of the Jones Libraries are making a $500,000 donation to the expansion and renovation of the 43 Amity St. building, drawing the money from what is known as the Woodbury Fund.

Trustees for the Jones Library unanimously accepted the financial gift on Dec. 20 from the Friends. The money will go toward the $46.1 million project that will expand the building from 48,000 square feet to 63,000 square feet.

“It’s just amazing. Amazingly generous,” said Austin Sarat, president of the trustees, before the vote by the trustees to accept the Friends’ gift.

Sarat said the Friends are among those who best understand the library’s needs.

“A thrilling vote of confidence from the group that I think knows the library as well, if not better, than any other group in the town,” Sarat said.

The money comes from the fund created in 2010 by a bequest from late Shutesbury residents Richard and Nathalie Woodbury, who regularly used the Jones and also made donations to the Amherst library.

Since the fund was established, it has grown to $791,560 as of Nov. 30.

Previously, about $182,000 from the fund was used to refurbish the building’s large meeting room, now known as the Woodbury Room, where numerous public events are held.

When the fund was initially created, following Richard Woodbnury’s death in 2009 and Nathalie Woodbury’s death in 2008, there was brief squabbling among factions at the library, with concern from trustees, who manage the Jones Inc. endowment, that they wouldn’t have control over the Woodbury Fund. Library officials later agreed to set up a subcommittee to make recommendations for using the Woodbury money, even though that technically went against Richard Woodbury’s explicit instructions, and led to threats of legal action from the Friends.

In the years since, though, the Woodbury Fund has been largely without controversy, and has been used as a constant means of supporting programs and other needs at the library.

Sarat said the donation is yet another example of the way in which the Friends of the Jones Library are indispensable

Library Director Sharon Sharry said the Friends thought through how the Woodbury Fund could benefit the library. “I’m so grateful to the Friends work they have put in, since 2012, in overseeing the Woodbury Fund,” Sharry said.

Sharry quoted the late George Goodwin, a former member of the Friends group, saying the Woodbury money would be used “to do more such bold undertakings.” Sharry said the expansion and renovation is such a bold undertaking and that a room named after the Woodburys will remain even after the building project is completed.

Richard Morse, a representative of the Friends, said the group was grateful to facilitate the contribution and appreciates the Woodburys. “We honor the intent and the spirit of the origins of the fund,” Morse said.

The donation from the Friends was brought forward by trustee Lee Edwards, who has co-chaired the capital campaign, and called it “truly thrilling, exciting and wonderful” before she made the motion to accept the gift.