The library was built by James Gibbs, for Edward Harley, in 1730, he also more famously designed St Martins in the Field in central London.
The library at Wimpole has over 10,000 books and was the combined effort of both Phillip Yorke, 1st Earl or Hardwick and Edward Harley. As far as I can tell anyway- I'm finding the history of the house quite confusing. The estate passed hands a fair bit, and wasn't kept within the family. Plus none of the people were particularly notable.
There is so much more sunlight in these rooms than in lots of other National Trust houses, which is makes it clear that the owners didn't view the library and the books as ornamental, they wanted to be functional rooms with plenty of light for reading.