Robert Graves' Goodbye to All That. It's his autobiography, and possibly my all-time favourite book. He describes fighting in WW1, which sounds grim I know, but your uncle might find it inspiring. The courage, and the matter of fact way he deals with death, is weirdly comforting. It's also full of wonderful anecdotes about his time at an Edwardian boarding school, his friendship with Lawrence of Arabia and Siegfried Sassoon, etc. He's just incapable of being dull. Almost every paragraph contains a fascinating story or observation.
Graves also wrote some brilliant historical novels. I don't know what era your uncle is interested in, but his I Claudius books, set in ancient Rome, are brilliant (the TV series is also a masterpiece). And he wrote a novel set during the American revolutionary wars, one about John Milton (17th-century ish) and one set in ancient Greece.
As for history books, Terry Jones wrote one about the Middle Ages. It's called Medieval Lives and is a light, humorous, entertaining work, filled with lots of photos and illustrations. It's a very easy read and would be perfect for someone in hospital.
Peter Ackroyd is a very readable historian as well. His book on Dickens' London is great. He also wrote a book on ancient Greece and one on ancient Rome. I haven't read them, but I'd assume they're good.
Andrew Roberts's book on Napoleon and Wellington is interesting. And anything by Max Hastings or Antony Beevor is worth reading. Both are historians with wonderfully clear styles.
My mother loves C J Sansom, who wrote a series of novels set in the Tudor period. Or how about the Sharpe novels, set in the Napoleonic wars?