I recently read Bertrand Russell's autobiography and loved it. He's a wonderful writer – clear, funny and wise. He also lived an interesting life and met countless famous people, from Wittgenstein to Lenin and Einstein to T. S. Eliot.
I love Robert Graves' Goodbye to All That. It's a masterclass in how to write. Not only in terms of style, but also content – he's just incapable of being dull.
Claire Tomalin's biography of Thomas Hardy is very good. In general, I'm not a fan of biographies (I prefer memoirs and autobiographies), but I like Claire Tomalin. I keep meaning to try her biography of Dickens. I think she wrote one on Jane Austen as well. Richard Ellman's biography of Oscar Wilde is supposed to be a masterpiece. I'd also recommend Ray Monk's biography of Wittgenstein. His philosophy is way beyond me, but he was a fascinating character, and the biography really brings him to life. Wittgenstein's Poker is a great book as well.
How about Peter Ackroyd? He's written loads of biographies. His most famous one is on Blake, but I think he wrote one on Newton as well, and even on Charlie Chaplin.
I've always meant to read T. E. Lawrence's Seven Pillars of Wisdom (would be interested to hear if anyone else has).
Christopher Humphrey's book The Inklings is one of my favourites. The Inklings were a sort of drinking club at Oxford formed by C. S. Lewis and Tolkien during WW2. Charles Williams was another member (a sightly crazy fantasy/sci fi writer and intellectual). It's a great evocation of that world. It's also a very cheerful book. Lewis and Tolkien and the others really enjoyed life. They'd drink beer, chat and read out loud to one another. Tolkien read a lot of his work to them. I love to imagine these heavyweight Oxford intellectuals getting pissed and listening to Tolkien read them stories about elves and wizards!