Great question.
There are so many wine books (and I have written two of them

!
If your friend is getting into wine then what I found tremendously useful at the beginning was Hugh Johnson�s �Pocket Wine Book�. There is an annual edition, the latest for 2011 has just been published and it really is an encyclopedia you can carry in a handbag or man�s jacket pocket. So useful when you are in a restaurant faced with unfamiliar wines on a list. Pull Hugh out and you�ll get details on the style of wine and the best vintages and wineries.
For a full sized encyclopedia you cannot beat Jancis Robinson�s �The Oxford Companion to Wine�
And there is Wine for Dummies � which is a very basic introduction that covers all the points.