Best non-fiction book I've read

 This topic from Long and Short Reviews was an interesting one. I read a lot of non-fiction in various areas, most often Richard III or on America - I've just finished Kate Andersen Brower's books on the presidency and they were excellent. However the one that came to mind here was "The Brontes" by Juliet Barker.

"The Brontes" isn't a quick read - it's a tome, but Barker's research is in-depth and fascinating. I live reasonably close to Haworth and the Bronte Parsonage, so the Brontes have always interested me, and Barker puts a lot of effort into debunking a lot of the Bronte myths and bringing unknown facts to light. She also gives my favourite Bronte, Anne, her due by going into plenty of detail about her life and writing, which a lot of previous biographers tended to skip over.

So what's the best non-fiction book you've read? I'm always open to a new one.