British Library
The official book depository of the United Kingdom, holding a copy of every book ever printed here, and a wide variety of periodicals. No less than 150 million items catalogued and there is an unimaginable of shelving! Visitors may not use the library unless they have proof of id

The official book depository of the United Kingdom, holding a copy of every book ever printed here, and a wide variety of periodicals. No less than 150 million items catalogued and there is an unimaginable of shelving! Visitors may not use the library unless they have proof of id
Check the official site before travelling; hours and prices can change.
More museums

Sir John Soane's Museum
Free, tiny, deranged — Soane's own house preserved exactly as he left it in 1837.

The Wallace Collection
Free national museum in a Manchester Square townhouse — armour, Boucher, the Laughing Cavalier.

The British Museum
Free, vast, civilisationally heavy — go for one room, not all of them.
Source: wikivoyage