Context? Looks like it's in an office, or is it a family room? What is your style, what else is in this room, etc.?

To maximize storage capacity, check to see which shelves, if any, are moveable. Organize books according to height, and keep the shelf height close to their tops. All flotsam needs to be organized into covered containers, labeled, and placed in priority of usage. If you have space left, you can stage minimally: frames of similar style to keep the look clean, a few lesser used books on their sides, a sporadic decorative item. Allow for blank spaces, if you can. This looks to be a working area(?), and so function should be prioritized before staging: the idea should be to keep a clean, rather than staged look.

Hi Danielle, these shelves provide a lot of storage, but to me anyway would be a nightmare yo keep organized.

Oops hit submit button by accident before I finished my last comment, lol! What I suggest is to add some film to shade the glass, I attached a sample of some below, there are so many patterns. It's as easy as putting on a decal too. That would help with the visual clutter a lot. For the open shelves, I suggest buying numerous boxes, baskets etc and group items in them as you would use them. I think it would be lovely painted to match whatever style you are going for.
http://houzz.com/photos/7285348

this looks nice
https://www.houzz.com/photos/2014-designers-own-home-contemporary-home-office-dc-metro-phvw-vp~13779638

or this

That is a mess! I would store most of that and rotate it every few months or when you start to miss stuff. Perhaps put up a light?

Well, that sounded rude, sorry. I mean it just needs a little work rearrangeing and making large areas of space between things.

Organize the books and big stuff in groupings. Some straight up and some on their side. Sculpted bookends make for a nice focal point at the end of a run of books. Plus it keeps them standing straight if the shelf is only partially full.

For the small stuff get bookshelf baskets and tins/bins from Ikea (if you have access to that store). Alternatively try Home Depot.

Then get some artwork and pottery like in anne dee's example pics. It'll be beautiful before you know it.

Question: Overwhelmed with office stuff...What should I do?
Answer:
I see many clients with the same problem...and as the new year is here...I am sure this will be one of the questions for the season. People want to clean it up, and purge so they can feel a sense of relief and renewed spirits! This bookcase was built long ago and most likely the person who built it did not even fill it up...but now as time goes on we gather more and more items and have a very hard time getting rid of things we do not use or need.
In an attempt to help you with this problem, I believe you could do a couple of things:
1. first purge
2. Then evaluate what is left and reorganize your items
3. Then if it seems there is still too many items, you may need to decide to rebuild the unit or at least remodel if possible.
a. If you choose to rebuild it...you will be very surprised at just how much of an improvement it will be to your entire situation.
b. If you choose to remodel...how about taking out the glass and having wood panels inserted so that it will hide everything.
4. My last tip; to make this more attractive...books mixed in with some accessory pieces will make it seem a little more relaxed and more pleasurable to the eye!

Painting it might help if that is what you choose to do...but the most important feature I see here is the organization.
If you decide you would like more ideas or help from me and my company, please feel free to contact us at www.interioraffairs.com or call us at 714-970-8000.

Here are some Houzz pics of some well used and seemingly loved bookcases.
http://houzz.com/photos/507619
http://houzz.com/photos/998064
http://houzz.com/photos/147598
http://houzz.com/photos/2108298

Thank you for all your ideas and comments- love this forum!

Thanks for all your ideas! I've been busy and making progress - now i need a desk to complete the home office and potentially paint the cabinets - what are thoughts on Annie Sloan paint?

I would say to paint the wood white just because the white would make it look more modern and a little but less dated

How is the progress with your room/bookshelves? Personally, I wouldn't paint it white. We have the same crowded bookshelves like you because we are both teachers and big readers. Best thing we did was just keep purging!

Totally agree with ASVInteriors that some nice storage products in an accent color might be just the thing to give these shelves a streamlined and

fresh look. Also- if you have a ton of books, books by color can make a space look more organized. If you want to reference by subject, obviously that won't work but if everything is fiction, for example, I would definitely give color a try.