Wooden door absorbing moisture, what can I do?

I've been through this at home and it's annoying for sure. If the swelling isn't too bad, you can recover it: take the door off the hinges, let it dry in a ventilated spot for a few days (away from direct sun, which warps it), and then sand down the edges that are rubbing against the frame. If it gets really bad, then you just have to replace it, because wood that's too swollen doesn't go back to 100%. In the meantime, if the door closes but with difficulty, lubricate the frame with silicone and put a dehumidifier or even a container with coarse salt nearby to control the humidity in the room. Prevention is easier than a cure - install a sill or take care of your window seals.