What a ridiculous question. As if, out of the world of literature at our fingertips, I could possibly pick one favourite book. You could at least narrow it down to a genre, although choosing a favourite crime novel would also be difficult, seeing as how I've only ever read crime books as a result of desperation at having absolutely nothing else to read.
Anyway, I suppose I could answer this with the list of books I took to Senegal with me; luggage allowance restricted me to only a couple so I chose 'The Lost Art of Keeping Secrets' by Eva Rice, and 'I Capture the Castle' by Dodie Smith. They were chosen on the basis that I thought I would be able to read them over and over for a year and not go too mad. Which was true. People who know me well might be wondering about the lack of Jane Austen in my luggage - that was due to my theory that any community of more than 3 ex-pats would probably include owners of classics (fyi I was right. It also included people with almost all Georgette Heyer's regency romances - I call that a win.). I also didn't take any L.M.Montgomerys, partly because they're mostly series' and you can't just take one of a set, but also because they're all available online.
Right now my brain is filling up with books, all of them shouting 'pick me! pick me!' Maybe I should write my own version of that BBC top 100 books... It would definitely include the Harry Potters (sorry to those of you out there that think they're evil but I love them), Narnia (oh, Narnia!), LOTR, some Sproul (not fiction but fabby), Tozer (ditto), Enid Blyton (just finished re-reading Malory Towers, great nostalgic reading), as well as all the ones I rejected in the 'favourite film' post for being better as books. Man, this is turning into an epic ramble...
Also, for those who are wondering why I didn't just pick the Bible and be done with it. Don't worry, I still love it! But I've always held the view that it's totally cheating to pick that in these situations. Akin to those people who, when asked what item they'd take to a desert island, answer 'a boat'. Gah! Aside from that, I wouldn't really put the Bible in a list with Jane Austen. It has a category all of it's own, a much higher one!
Finally, completely gratuitously, look at all my books! Okay, only about half are mine, but the hallway in my flat is lined in bookcases filled with our (mine and Beth's) books and it makes me happy! Sometimes I'll just look through the open door of my room at all those books and heave a sigh of contentment...
