Previously unknown sketches from the early years of English painter Thomas Gainsborough have been discovered tucked away inside a royal library, Rebecca Jones reports for BBC News.

For more than 100 years, the 25 black-and-white chalk drawings were held in a leather-bound​ book that had sat on the shelf of Windsor Castle's Print Room, Jones writes. The book had been mislabeled as work from artist Edwin Landseer, who is best known for sculpting Trafalgar Square's iconic bronze lions.

"It's the very best collection of Gainsborough's early drawings in existence," historian Lindsay Stainton, who first noticed the sketches, tells Jones.

In the late 18th century, Gainsborough was one of England's most-acclaimed portrait painters, even courting special favor with King George III and his family, according to London's National Gallery. But though the artist's fame and money came from his portraits, his real passion lay in landscape drawing, art historian James Hamilton, who is publishing a biography of Gainsborough next month, tells Jones.

These newly discovered drawings illustrate that interest. Gainsborough was in his 20s when he made the landscape sketches, which take inspiration from the trees, lakes and animals of his home county.

The mislabeled sketches first caught Stainton's eye in the 1990s, reports Dalya Alberge for the Guardian. It was only after she came across a sketch that resembled "Cornard Wood," a Gainsborough painting hanging in the National Gallery that she became convinced of the artist's authorship. When she had the sketch superimposed on the 1748 painting she knew for sure.

While the Royal Collection did not discuss the value of the drawings, other Gainsborough sketches have sold for more than $1 million, writes Naomi Rea for artnet News. One such sketch auctioned in 1991 had been rediscovered decades after its owner, art collector Harry Elkins Widener, died on the Titanic, according to the Desert News.

More recently, a copy of "Cornard Wood" thought to be an imitation, was authenticated by an expert on "Antiques Roadshow" in 2009, writes David Millward for The Telegraph. And just last year, according Chloe Leonida at artnet News, the earliest known commissioned portrait painted by Gainsborough was sold unknowingly at an auction for $3,180 before it was identified.