It is normal that you need to speak to Antopar to get him to unlock the library each time yo wish to enter.
If he is inside the library, that is a bug, with a couple of workarounds: the procedure in the topic
armory trick in act 1 can get your characters through the door to speak to him, or you can teleport him out into the hall (
Teleporting anywhere (via hex editor)).
For the latter, if you are not comfortable hex editing, zip your latest save and email it to support@larian.com and I can take a look at it.
