Thanks for your all the replies.
I do read to my 6-month-old not because I want him to understand the words or pictures whatsoever I simply want to develop this habit on him from a very early stage (reading is a super good habit!). When I read to a baby (same to my older one when he was a baby) I held him on my lap and read to him. I agree that we do not need to mind the content of the book, simply let the baby hear your voice and be physically close to you is good enough.
Unlike my older boy, my younger one still does not have a sleeping pattern, i.e. he can fall asleep at 9pm or 10pm or still awake at 9ish. At the moment when both of them are awake, I would make them lie down with me in my bed and I read a book suitable to my older one. In this way, I do read stories to my older boy but let the younger one "feel" the reading and hear my voice.
When do I "read" to my younger son in a proper way (i.e. hold him on my lap and show him the book in front of his eyes)? It's when my hubby plays with my older one. As the attention span of a 6 months old is really short so I read to him 5~10minutes is enough, in my case.
I know, when the younger one gets older I need to make a better arrangement about 'bed time stories', and I think, after reading all the advices here, I would start with a simple book so that the older one can be a 'little teacher'; then I will move on to read a more complex story for my older boy. Usually, my older boy will not be satisfied with just one book.
So, let me see how it goes. Reading is great. I read to my boys whenever I can't think of anything interesting to do!