Are particleboard or MDF bookshelves strong enough to hold heavy books?

It depends on load and span. Across specialist communities like r/HomeLibraries, the consensus is that mass-produced MDF and particleboard shelves can sag over time under dense book loads, especially on wide unsupported spans. Reviewers repeatedly advise keeping heaviest books on lower shelves and using any included anti-tip straps. Listings here quote per-shelf ratings ranging from about 22 lbs (VASAGLE) up to 60-70 lbs (Bevfint, SUPERJARE), so check the rated capacity before loading a shelf with textbooks.