From the Stowe 1848 Sale Catalogue:

The Library of the Right Hon. Thomas Grenville, one of the most valuable ever formed by a private individual, was once destined to have occupied this room. A celebrated architect actually received Mr Grenville’s instructions respecting the alterations of the presses for its reception; and about ten years since, the right honourable gentleman expressed to the late Duke his intention of bequeathing his collection of books to him and his heirs. This intention was, however, subsequently changed, and within a few months of his death, Mr Grenville, by a codicil of his will, gave them to the British Museum. Mr Grenville’s Library was the result of a continued and unwearied pursuit of nearly fifty years, guided by a very extensive knowledge of ancient and modern literature, and by a familiar acquaintance with rare and curious books. The entire Library consists of about 20,000 volumes, among which are many of the earliest and most curious specimens of typography; first and best editions of the Classics; the scarcest Spanish and Italian Poems and Romances; many books printed on vellum of extreme beauty; a range of English, and more especially Irish, History—perhaps unrivalled; and an assemblage of early Voyages and Travels, from the original editions of Marco Polo and Contarini; Columbus and Vesputius, to the collections of De Bry, Halsius, Hakluyt, and Purchas, forming such a complete chain of uninterrupted information on the subject as no other library can furnish. In no branch of this collection is anything superfluous to be found; while there is a sufficiency of information upon all. With the exception of George the Fourth’s gift of the King’s Library, this is the most magnificent donation every made to the British Museum, having cost the late owner about £50,000. It is perhaps not improbable that Mr Grenville, with that wisdom and sagacity for which he was so eminently distinguished, foresaw the ruin impending over Stowe, and was therefore, induced to alter his original determination:—

“The sunset of life gives us mystical lore,
And coming events cast their shadows before.”

A most excellent Catalogue Raisonné of the Library has been compiled and published by Messrs. Paynes and Foss, under the title of “Bibliotheca Grenvilliana, or Bibliographical Notes of Rare and Curious Books, forming part of the Library of the Right Hon. Thomas Grenville. 2 voles. 8vo. London, 1842.” Mr Grenville died in December, 1846, at the age of ninety-one, at his house in Hamilton Place, after a very short illness.