How do I clean the glass doors?

Use a soft cloth and a gentle glass cleaner. You don't want liquid running down into the wooden putty lines, so spray the cloth rather than the pane. Be gentle when wiping near the thin astragal bars. The old crown glass is thin and cracks under firm pressure.

We have an uneven floor in our old house, so will a big antique bookcase lean forward and stress the joints?

It isn't unusual for large cabinet furniture to need a little help sitting level on old floorboards. You should use wooden shims packed under the plinth or feet. Ensure the carcase is square before loading any books. If the unit twists, the glazed doors bind in their frames and might warp.

Can old shelves take modern hardbacks?

Solid mahogany or oak shelves are tough. They'll easily hold heavy art books without trouble. If the span is wide, you'll want to distribute the heaviest volumes towards the outer edges near the side supports.

We are moving to a smaller house and want to know how easy it is to take these big pieces apart for the movers?

Almost all large examples were designed to be dismantled. The cornice lifts off, and the top glazed sections usually separate from the deeper base cupboards. On breakfront models, the side wings detach from the central section. Check the backboards for original screws holding the carcase sections together. Label the original screws and tonk fittings when you remove them. The hardware is difficult to match if lost during a house move.