My IKEA bookcase collapsed at 31 books, and that number finally made me drop hardcovers from my club picks
I used to argue that hardcovers were worth the weight for our monthly pick at the Ferndale library meetup, until my Billy shelf gave out right at book 31 last Tuesday and I sat there surrounded by 40 bucks of bent dust jackets, so now I'm the guy pushing paperbacks and half the group says I've ruined the whole feel of it.
That line about the Billy shelf giving out at book 31 is real, those things are rated for way less than people think and the spec sheet hides it. I had the same thing happen and the fix was cheap: get a thin sheet of plywood cut at the hardware store and lay it across the back, then screw it into the shelf sides. Also spread the weight out, put the heavy stuff on the bottom shelf and keep the top row for paperbacks. Your group will get over the feel thing once they don't have to buy new dust jackets every month.