Since I was a child every time I've visited an old stately home [UK here] such as Castle Howard I've been curious about the many hundreds of books that are on display in the old wooden bookshelves and whether, if I was able to, they are still readable or whether the print has faded over the centuries.
Are these books now just 'ghosts' of what they once were, a spine showing titles such as 'The collected speaches of Lord Fizzle, vol. XI 1835-1837' but inside pages which are too fragile, too delicate to be read, and whether these books are of interest to historians or if all useful information has already been extracted and preserved digitally elsewhere.
Sometimes these books are primary sources or compiled anthologies of speeches and other parliamentary documents / discussions of relevance, which can be fascinating to read as a contemporary historian. Some of these collections are in national/university libraries or are available via RACER and therefore aren't hard to get hold of.
Because a lot of UK aristocrats participated in the house of lords or had a vested interest in UK politics, their vast collections often reflected this participation. As a social historian who also deals with eighteenth century British politics and the aristocracy, I find personal collections intriguing and full of details about the interests of the Earls and Dukes who curated these collections (whether they were a whig or tory, their thoughts on certain subjects, how their collection has been handled and carried down through generations etc).
The books themselves might be a bit fragile but readable if you've had training handling old texts; it depends on how used they are or how they've been kept. To be perfectly honest, a lot of these collections contain the classics - The Decline and Fall of the Roman Empire by Edward Gibbon, various books by Churchill, Lewis Namier etc. which are valuable in a historiographical sense if you're interested in looking at the progression of a particular field of history. They would be mostly outdated for historians to use in their analyses unless they are a collection of primary sources. There might be rare editions of fictional books or early versions of Shakespeare/Dickens. Other books might be rare editions of property management and agricultural practices. What is extremely fascinating are the scribbles and notes in books that sometimes appear. You can begin to piece together a person (or multiple people) which I think is fantastic.
source: personal research, MA in history.