This is badass… step-by-step instructions for designing and building
your own hidden bookcase-door. DIY means, of course, that you don’t have to trust anyone else with the knowledge that it exists (critical, since it’s not exactly “Kerckhoff compliant”). And it also solves the problem of door-opening scuff marks on the floor by raising the door some height above the floor.
(The guy even uses Festool, the German toolmaker I mentioned earlier. Not just a plug… hidden anything is where precision matters.)
(too much to quote)