I know it doesn't appeal to everyone, but one part of the book writing process I really enjoy is research. For example, when writing my book about pioneer moving picture photographer Eadward Muybridge (The Man Who Stopped Time) I spent many a happy day in the local history room at Kingston-upon-Thames going through box after box of their Muybridge archive, handling original letters and photographs by the man himself as well as many photocopies and duplicates.
Anyone interested in really getting into the head of Charles Darwin might like to know that those nice people at the University of Cambridge have put scans of Dawin's personal library online. So far 330 of his books have been scanned with around another 400 to go. (He had 1480 books, but he didn't write in all of them.)
Now these aren't Darwin's notebooks or anything so personal, they are books by other people, so you might think 'So what?' But Darwin was a top notch defacer, a librarian's nightmare. He scribbled notes all over the books he was reading for research - and that's the point of these scans you get the Darwin touch, and an insight into how he reacted to the scientific books he was reading.
If you are writing about Darwin, this is a wonderful resource. But even if you aren't, it provides a fascinating glimpse into the mental processes of the great man.