fixing a margin

I got an drmn-free indy published book in mobi format. It has a really wide left-hand margin.

In Convert Books>page setup, I have margins set for 5 pts, but this is much more than that. Running a convert books doesn't do the job.

I converted to epub and exploded the book, but the css doesn't list a huge left margin.

I've never gone in and edited a book before. Can someone walk me through the steps needed to get rid of that huge left margin?

thanks!!