I would suggest that you try to find a non-standard bookcase. Maybe a selection of small wooden trugs or crates stacked to make a feature rather than just a boring bookcase. Another idea is that these crates could be washed with a light chalk paint to lighten the corner. Looking at your photo I would suggest placing the lighter picture above the TV instead of the darker mounted one. This might lighten up this area further.

lots of people have had a good input on colour schemes - colour is a very personal choice .... whether you go for classic rich colour or something more neutral with longevity ... The one thing I would really consider doing is to remove the shelves from either side of the fireplace or at least make them shallower in depth and definitely go for a statement wallpaper or contrasting colour on the fireplace wall. this would make a real feature in the room and less oppressive, opening it up - the existing shelving as practical as it may be really dominates. I appreciate it is probably more of a case of practicality. Also depends on what your office is trying to do for you .... do you need a showcase office to impress visitors, are you there all office hours, ..... colour can create an ambience a mood, relaxing you or giving you a headache ..... ! Most interior designers refer to a certain colour range on the market (F&B) all their colours are quite relaxing even thought they me be deep colours check them out and use them as a starting point - you can always adjust the colour slightly by getting one mixed specially if they are not quite what you want. Oh and on the point of atmosphere - think I would be inclined to add neutral wood to warm up the room ... the floor looks good for sanding down. Good Luck and please post the results.

You could do as above and trim around an 80 cm unit and give a real built in feel or as the shelves are adjustable you could fit two 40 cm units buy some extra shelving packs and just drill the holes out on both internal sides of both units and cut down the shelving to fit the centre column, I would still put some smaller trims round this option to just to make it bespoke!

Lovely link thank you carolins. I have spent hours looking at IKEA hacks. HI minnie and no I dont do DIY, a terrible at it sadly! The alcoves are just over a metre wide which is tricky with billy bookcases, the skirtings are fab, they are huge, around 30cm tall and moulded and there is a picture rail too. I would have the shelves up to this point. There are sockets at the back of the alcove so I need access to these so any shevles would have to be fitted above these. I did wonder about doing sth like this (using two sets of billys with a dividing upright in the middle to fill up the alcove width) but the shelves are a tad shallow and am worried it might look a bit cheap....? http://www.ikeahackers.net/2015/06/billys-alcove-alchemy.html