I still read lying on my tummy in bed, it's great. But I find that I'm tending to roll over onto my left side no matter how I go to sleep - even when I fall asleep onto my book, I'll awaken on my left side. I usually go to sleep on my right side, but it does make forcing DH to give me a cuddle in the morning easier if I wake up facing him. I think your body knows what to do, so if you don't move from your tummy it must be safe for you and the baby.