I'm halfway through John Stott's 'Why I Am A Christian' and it is very good for an introduction into basic concepts of the Faith. I plan on giving it to a friend who has told me many times, "I wish I could believe as you do, but I just can't."
Maybe she would enjoy 'A Praying Life' and 'A Loving Life' by Paul Miller? They are easy to read, very helpful, and I enjoyed them very much (and hope to reread the first soon). Despite disagreeing with some things here and there, I think they are very valuable.
I have also loved biographies of women from reformed traditions: their witness to the sufficiency of God's word for our life of faith (even in very hard times) has been a huge blessing to me -- books by Helen Rosaveare, Darlene Diebler Rose's 'Evidence Not Seen', Corrie Ten Boom's 'Hiding Place'.
Your wife might also enjoy the books by Faith Cook that are composed of a number of shorter biographies (like 'Seeing the Invisible').