The required design of the enchantment room bookshelves doesn't look at all like a real library. The library in Fortresses is much more like it.

But now with a few pistons and some redstone you can have a proper library and enchantment room all in one. Just flick a switch to change between them. The two remaining fixed bookshelf blocks are for those who wish to fit a hidden entrance or a storage chest. Only two blocks high below floor for ease of fitting into existing buildings.

The lever doesn't interfere in enchantment levels or function as that part of the wall isn't needed to access level 50. It can be moved further back or replaced with a pressure pad if needed.

If I get some diamonds I may upload my other enchantment room with the hidden door.