It looks like the glue from the paper still needs to be removed….go to a hardware store and get the right remover… if there is still a residue… try a light sanding before adding stain or paint

Thanks for your response. It wasn't the glue residue, the peeling off is from the door only. Not sure the best way to explain it..Basically it's coming off the door. I put a clear tape and pulled it and got the same effect.

Should the next step be sanding the door? Can I make it the same throughout without painting (so the entire door would look like the bottom left of pic 1)?

Oh no… it sounds as if the veneer has been pulled off the door?… not sure there is a fix for that…if it is a solid wood door sanding will be worth a try… but if it is veneer a new door may be the best option…veneer doors are not that expensive…measure the door and try calling suppliers for prices…

Try sanding first … it is hard to say what is possible just from this picture

I would get a new door also. I'm not even sure that was a real wood veneer. Suspect that it is more like some kind of faux woodgrain plastic overlay. Close up picture?

If that is exposed wood, you could go ahead and peel off or lightly sand away the remaining wood look and either clear finish or prime and paint.

Carefully sand because you don't want to burn through a veneer layer (if it is wood underneath.)

Thank you all!

Is there a specific sandpaper I should use to even out and get the door to the look in bottom left of first pic?

And here is a closer look of the back of door

It looks like the wood grain on the door was a paper type laminated finish, and the wallpaper has pulled it off. If that is the case, there is no way, that you will ever get that door to look like the one that hasnt been wallpapered. You can , however sand it and paint it. Since the side that is on (hopefully), a hall side, still matches, all other doors, I would paint it, because, chances are you wont find that door anywhere to replace it, so they all look alike.

I can't tell what that material is. Sure enough has a wood grain texture. Sanding will remove the texture so you should probably peel off the remainder, skip the sanding and prime it with a good bonding primer like Insl-X STIX .

It looks as though the door opens into hopefully a bedroom or bathroom. If so, you can get away with having just one side painted and not have to paint the other doors.