It’s a library. There’re so many books in there. Didn’t get a chance to read them all.
Check out all the important historic documents. They have a great exhibition now on Shakespeare.
Spread across 4 floors and a glorious atrium, the British Library is seriously the best place in London to work productively. And it's certainly no secret. I frequent there for long sessions of laptop work and have counted over 1000 people - students, freelancers, academics - setting up work stations there every day. The vibe is enterprising and motivating, to say the least. There's even a nice cafe and restaurant and an exhibition hall if you fancy having a break.
Fascinating place for those interested in history. You can see original documents from around the world, including an original version of the Magna Carta and original prints of Shakespeare. Regular exhibitions highlight their treasures. If you can become a reader you have access to the stacks of one of the great copyright libraries of the world.
This may well be the most important and accessible library of its kind. Also probably the best run. Slightly suffers from the policy of letting A-level students use the revision rooms to revise (i.e. you don't have to be reading something from their collection to grab a seat), but the manuscripts room and rare book rooms are unparalleled. Paradise for academics.
Don't pass up a (free) trip to the British Library Treasures Room, especially if you're a history buff. Plan to spend about an hour to see the original Magna Carta, Shakespeare manuscripts, Leonardo da Vinci documents, Galileo's first scientific publication, a page from the Gutenberg Bible and hundreds more priceless original documents.