Just a spirit. They're everywhere. Shadows/shadow people are black, like shadows, not grey.
Unless you feel threatened or feel its harming you (feel ill, drained, or moody when you see or sense it) then just let it be. You did the right thing by ignoring it. Who knows? Maybe it was interested in your book! Haha! Or maybe he's a guide checking up on you.
I always suggest keeping your home cleansed regularly (to prevent negative energy from building up and attracting negative spirits) and wearing an amulet (it could be as simple as a ring or pendant you wear everyday that you empower with protection through ritual). This will give you peace of mind that even when a spirit invades your personal space, they can't do any harm to you and you can continue on with whatever you're doing, ignoring them if you wish.