mdf will sag under its own weight if the span it even quite small
personaly assuming half inch i would go for 24 inches max for heavy books the shelve will still sag slightly thats assuming they are fully supported on three edges
if you support the front edge with 2"by 1"planed timber with a half inch square rebate you can glue this on small profile showing to give a good bit of support
turn it round with large profile showing you can increase the span or the weight by about 60percent or a bit of both
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