Rothermere American Institute

Not only is this a modern building in Oxford that is actually aesthetically pleasant, but it is also my favourite library IN THE WORLD bar none. The library itself is called the Vere Harmsworth — to be honest I don’t think the building itself is used for much undergraduate stuff, mainly conferences and postgraduate lectures, but the library is superb — a great place to work and so well laid out you could literally just cry with joy. One day I plan to rearrange the Hamburg Staatsbibliothek to make everything just like the Vere Harmsworth. An ambitious task, sure, but it would be worth it. They would thank me afterwards.
The building itself is dedicated to American Studies/​History etc. so this is the literature it carries but you can also order stuff out of the Bodleian to here which is an excellent feature. And the fact that it closes at 7 is a good excuse never to work in the evenings.
Minor annoyance is that it is barely ever open at the weekend but this is compensated for by… well, by everything else.