The Company of the Preachers by 
Concise Encyclopedia of Preaching by 
The New Interpreter's Handbook of Preaching by Commentaries are quite useful when you need to explicate scripture in a sermon and need some further background. Just remember that different commentaries have different audiences; some technical commentaries have academics as their intended audience, and you may have to do more work in adapting their information for a general lay audience. Listed below are some basic expository and application types of commentaries to give you an idea of some commentaries that might work in your sermon preparation.