Learning it from books is fine for most emergency surgery in my experience, so why not give it a shot...
Seriously though, I'd only go down this route if you feel you'd have no option. And in fact, even then, I fear it could do you more harm than good.
Example: Mofo with big knife demands your wallet and iPhone.
Parallel Universe 1: You give him your stuff. Live to mope. Bruised pride, wounded regrets at not getting round to reading the Dorling Kindersley book on superninja jap-slapping skills. Superficial injuries to ego.
Parallel Universe 2: You've read everything Ashida Kim ever wrote and feel emboldened enough try using it. Endex for you.
If you're lucky there's a surgeon waiting for you who hasn't learnt it from cut-by-cut illustrations in a book.
I doubt a book can ever teach you the confidence to defend yourself in a fight. Personally I feel my sole source of confidence in that respect comes from knowing I don't harbour any undue compunction, hesitation or compassion which hamstrings my ability to fluidly impart even lethal force should it feel essential for the protection of my life. Somebody else might say it comes from the ability to do a particular set of moves or look good in a gi, but as far as I'm concerned, no book does that for you, with the possible exception of a draft of Steven Griffiths' PhD thesis.
So in my opinion the best book on self defence ever is this http://bookshop.blackwell.co.uk/jsp/wel ... 1177945481
Finally, a thought for the day, grasshopper.
in a situation where I actually need to fight...
There are many more situations where you just think
you need to fight than there are situations where you actually need to. And the frequency of the former increases in direct proportion to the weight of teach-yerself kung fu books on your shelf at home.