Subjects (eg. billboards, lighting, decoration, detailing) are in bold text, while important points (too organized/cluttered, too small/large, could be brighter) are in italicized text. The plain text elaborates on the important points. Once in a while, I'll throw in links to pretty pictures to help emphasize points or give examples - they'll be much more useful than those two links, no worries.

So uhm, I'll leave commenting on lighting until last since I'm sitting in a very sunny area and using my hand as a shade just to see my comp screen (and it's on full brightness)! I had to open your pictures in photoshop and turn up the gamma by ungodly amounts just to see the contents - hope you don't mind c:

That text at the top of your first screenshot, get rid of it :p Seriously, it's easy to do and looks way more professional. I believe you can turn off hints in the options menu from in-game, there's a way to turn off the screen's darkness effects too (I can't remember how though, but it doesn't seem to be a problem with your screenshot anyways).

Alright, second is: what is the room's purpose? Every room has a purpose - even in a giant mansion :p To me, this one seems like it could be a storage room (lots of shelves, boxes, not really designed for spending time relaxing), a reading room (usually dim light, a fireplace, comfy chairs, lots of books), or maybe a small, personal library (lots of books and shelves, comfy seating, well decorated - probably designed for showing off knowledge of some sort). Might want to clarify this a bit, because it currently looks like a mix of all three. Objects - like the chandelier - look out of place and mismatched due to the lack of clarity.

The current placement of large objects is actually really good because it allows the player to move freely through the room without getting stuck on items (although I have a thing for cluttered rooms - like Hardarm's maps).

As for the rest of the objects, you might want to add a human element. For example, book shelves and carpets being off-angle by 2 degrees, a few of the single book entities lying sideways/crooked on shelves, piles of books by the chairs for reading, books put back in their spots the wrong way around. Think of how humans interact with their environment. With shelves, most people are too lazy to reach for the stuff on the top shelves and too tall to bother paying attention to the bottom shelves - so the stuff in the middle (particularly around chest-level) is going to be less organized (or even missing in some spots) since it's taken out more often. Don't be afraid to go full Sherlock on the room, either (I've done that before, lots of fun! People that notice the detail appreciate it and those that don't are still more immersed in the environment because of it) :D

Your detailing isn't too bad currently, could use a bit more work though. I noticed there's a wine bottle on the floor near one of the carpets - you'd probably expect a bit of wine to be spilled there with it, maybe some goblets to drink out of (unless they were planning to finish off the bottle in one go - crazy drunk :p). Since the bottle stayed on the floor and there are cobwebs in the corners, I don't think the people using that room were being very tidy. It wouldn't hurt to throw in some static dirt decals on the ground and fog particles in the air to simulate dust (and give a bit of depth). Sort of like this sexy map (also Hardarm's) or this one (mine - all mine >:D).

It's still way too bright where I'm sitting for me to see your screenshots without messing up the gamma, so I'm afraid I can't make any comments about the lighting right now. Hope I was still able to be useful! :D

Oh and one more thing - I know it's a lot of stuff I commented on, but don't get discouraged c: it's more of my awkward way to encourage people to improve :p