If you use biscuits, the glue will be binding the biscuit to the wood inside the melamine, so you don't have to worry about the glue sticking to the melamine. If you want to use screws, you can get little white plastic screwcovers
- some stick directly into the philips head of the screw, some are basically white stickers, or you could countersink the screws and get white plastic plugs. Try
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white plastic screw covers. Will it be strong enough? I think your design sounds stronger than many of the pre-cut, assemble-at-home MDF bookshelf kits you get from Target or whatever. If you're really worried about sagging, my only suggestion would be to put a vertical strip of white melamine (or whatever) under each shelf to help support it, or on the front of each shelf like a face frame. Remember to use good dust collection and wear a mask when cutting MDF - not very good for you. Good luck, Andy