Since I first heard it recommended by Archbishop Fulton Sheen, I've been a regular user of William Barclay's commentary on the New Testament. I find it especially helpful for my daily Mass homilies, in the way they give a historical background to the people, places, and things found in Scripture (giving Mass attendees a tiny insight into the reading's context). Abp. Sheen does give a caveat: Barclay is a Presbyterian Minister, so his perspective on certain Scripture passages (John 6, Matthew 16:18, John 20:22, etc.) is not particularly Catholic. But by and large, the books are a great resource for anyone who leads a bible study or preaches regularly.
This one I found neat, for today's First Reading from Revelation, about the scroll being sweet like honey:
"It may well be that behind these words lies a pleasant Jewish educational custom. When a Jewish boy was learning the alphabet, it was written on a slate in a mixture of flour and honey. He was told what the letters were and how they sounded. After the original instruction, the teacher would point at a letter and would ask: 'What is that and how does it sound?' If the boy could answer correctly, he was allowed to lick the letter off the slate as a reward!"