I like the idea of glass panels with or without fabric. As far as fabric color, you will need so little you can probably do a few different colors and change them out occasionally. If you do glass panels you can pick a glass with some pattern, like reeded glass for example, that would obscure the cabinet contents a little.

Go to a glass place and look at the textured glass available...they can obscure yet add interest.

Thank you both for you suggestions.

Google searches found some wire mesh sandwiched between glass. While I like that look, the associated opinions indicated that idea might not be safe. Since the doors will be close to the floor, they might get knocked around by chairs. What do you think?

Anyway, I'll take your advice and look at textured glass available here

Depending on your decor and level of desired formality, you could skip the glass and just use fabric. In addition to standard yard goods, you might use speaker cloth.

Unnless you do soe pretty hefty chair slinging, wouldn't a cchair only be likely to bump the edge -- most llikely the frame, and not hard enough to break a sheet of glass unless you use something really delicate -- and textured glass is likely to be thicker than that. You could also use sheet metal in a color that works withthe room. Tooled or pressed copper?

Can you post a picture and then we might be able to give more ideas? A lot depends on the style of the house and bookcases.

I hope this works. I'm learning about using the Houzz interface. Sorry about the quality of the old pic, used a camera without stabilizer.

There are two of these bookcases. The doors would cover the bottom two shelves (30 inches high). Probably four doors to cover 50 inch I.D. span. The house is traditional. The floors are distressed oak. None of the furniture is part of a matching set, just dark woods.

The copper idea sounds good. Does it tarnish like cookware?

You are all so helpful! I really appreciate your efforts.

Could you take the back off the bookcase and use that for doors? If so that would look very nice. The back could be replaced with something very close and stain matched. Once the books are back in and the shadows fall no one will be the wiser.

Actually, if you take the back of for the panels, you could do the back in a painted finish and introduce a color there. Copper would darken over time and eventually could patina if it is not treated to prevent it. Copper and brass are often coated with a lacquer when the finish is intended to be preserved. You could probably just spray the panels with a clear polyurethane in the finish you want (gloss, satin or matte) if you want to preserve the as they are.

You see tin ceiling tiles painted -- you could also do that in a color -- distress them if that look works with your room. Hybrid idea -- take the panels glaze them to age them (metal showing with some paint or stain coloring where the paint distressed would be mostly paint showing with a bit of exposed metal) , then coat them to preserve.

The bookcases are attached to the walls. They were built in situ. No way to remove the back without dismantling the whole thing.

Oh in that case I would skip the door idea. Keep your eye out for antique (if that is your style) interesting wood artistically painted or stained and or other material box storage containers to house your stuff. Or, do something nifty out of something new.