But at the time, I didn't feel "proper" to comment the work of well-known authors.

I felt the same way when I started in online fandom. You feel as if you're introducing yourself and in a strange way intruding on people you don't know. It's funny because the reality is that online authors are generally delighted to hear someone liked their stuff, but online readers (many, perhaps most of us) go through that "Who am I to bother this person, even to say something nice?" phase. It was knowing that I wanted to pay a compliment that got me through the first comment, but I was uneasy, and quite relieved when I heard back from that person with "Thank you!"