Larry W wrote:

I do have a freestanding mdf bookshelf in an upstairs room with 42"
shelves. When they started to sag, I grabbed a few old oversize books
nobody was reading and crosscut them to height on the tablesaw, so
that I could jam them into the bookshelf near the center for an
additional vertical support.

Heck of a good idea. You could even tear out 50 or so pages and imbed a
wooden support.

As for finding "oversized" books, I often see free encyclopedia sets on
Craigslist.