Susanna,
Those particle board bookcases are a concern. I keep machines (as much as possible) on solid wood units. Even there the adjustable shelves, held with those little brackets, are a concern. But so far, so good. Books are heavy also, so if something is made to hold books, it should hold a machine or two. Avoid units that are too wide, and we should be OK.
Have you sewn much with your machines? I don't have a 66, but my 99 sews nicely. A favorite of the newer ones is an aluminum handcrank 201 from the 50s. Sweet machine. My newest is a 1980 Elna, great machine now that it has a new pulley.
Dianne in Colorado