The Guardian has a very interesting article on the Bodleian library in Oxford. The lede is that Bodleian is has found facilities to store some 8 million books, but the article also describes this incredible library:
The Bodleian, which opened its doors to scholars in 1602, incorporates the collections of several libraries dating back to the 14th century. Its collections include priceless classical and medieval texts given by Duke Humfrey, younger brother of Henry V. Among its 9m printed volumes and 10,000 medieval manuscripts, the library holds some of the rarest and most valuable books in the world, including four copies of Magna Carta, a Shakespeare First Folio, the papers of six prime ministers, the original manuscript of Frankenstein, the handwritten manuscript of Kenneth Grahame's The Wind In The Willows, Alan Bennett's complete archive (donated last year) and the theme music for the Vicar of Dibley, donated only a few weeks ago by composer Howard Goodall. It is also a copyright library – for over 400 years it has received a copy of every book published in England.
And later on, seems that storing that many books is rather difficult:
Thomas has, she says, been having nightmares about the situation, which
risked causing library services to judder to a standstill. At one
point, after considering 90 potential sites, she found herself gazing
from her office at the quadrangle separating her building from
Christopher Wren's 17th-century masterpiece, the Sheldonian Theatre and wondering how much people would object if she filled it in with a massive books store.