So far we have done some Nicholas King and John Stott neither of which has particularly settled well with us, and also the Screwtape Letters by CS Lewis, so it doesn't necessarily have to be Bible based.
Would totally recommend Michelle Guiness "Woman - the full story". Want to suggest it to my group in the summer (we do books from the Bible for 2 terms, then something else in the third term.
We found Making Room for Life by Randee Frazee a good one for prompting discussion, if a little (well very) American. It's quite unusual and got good discussions going without being particularly threatening.
Really found it valuable working through Galatians last year as it was a book I didn't know well - used Tom Wrights "Paul for Everyone" and another commentary as the starting points.