I like commentaries because they are themed. It's really a super-long sermon that's been put into print. But, like text books, there are way too many books by way too many authors, so I won't specify any that I use. And let’s face it; trying to find verses in the Bible around some specific themes is like trying to find all the different shades of blue crayons in a box where the colors were sorted alphabetically.
Anyway, like I said, I like commentaries because they are themed. That is their purpose, but you also have to deal with the author’s interpretations (not to be confused with translations). That’s why when I want to learn about a particular theme, I usually look for at least two books by different authors. If it is a controversial issue, I’ll try to grab three books but make sure at least one is Pro- and one is Con-.
I currently use a 4-translation Bible that has KJV, NIV, NASB and Amplified. It’s nice being able to compare the different translations side by side. My personal preference is the Amplified. I find it helps with understanding the full concept of what was being written. It even interjects names when a lot of he’s, they’s or she’s are being thrown around back and forth so you know exactly who is saying what to whom. In addition, by default, the Amplified also cross references many verses where God promises He will do something or, for example, when someone in the NT references something in the OT. One caveat though: this is one of those translations that take the “husband of one wife” verses and changes them to “husband of but one wife”. Since it’s my most recent area of study, it’s the only big thing I’ve noticed so far.