Another good book by Yancey is "Where is God when it Hurts." I think I first discovered the theological side of CS Lewis in that book because Yancey references Lewis quite a bit.
Another good resource is
www.biblegateway.com . I use it often when I am looking for a passage. esword is another good program to install. I have the cheap program wal mart sells installed on my computer, it has a decent reference set. Though I do wish it had the NIV version. That is what is nice about biblegateway, it has all the versions you could want to search.
There is no end to what you could put on your bookshelf. I have about 3-4 books on how to read and study the bible, something along those lines could be helpful. Confessions by Augustine is on my shelf, but I have only read a few pages into it. There are a lot of classical works that most people ignore, but these works could help deepen your understanding of God or at least Christian history.