We do not need any other source than the Bible. It was given to us, and is sufficient for us. When reading commentaries, study guides, or other educational books about the Bible, remember they were written by man. Many times those tools will be helpful in understanding passages or give good insight into a verse. Often times it can be easy to put too much of man's ideas into a book and not rely on what the Bible is telling us. So read those books, pamphlets, or articles with a bit of prayer to be sure you are receiving the truth. And always use the Bible as your guide.
"Beloved, while I was making every effort to write to you about our common salvation, I felt the necessity to write to you appealing that you contend earnestly for the faith which was once for all delivered to the saints."
2 Timothy 2:16
"All Scripture is inspired by God and profitable for teaching for reproof, for correction, for training in righteousness."
2 Peter 1:3
"seeing that His divine power has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence."