Enter this: the Mark One 'Kvinn' Wheeled Modular Bookcase.
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There is another, subtler bugbear of would-be librarians who yearn for order: that of expansion. Dedicating a shelf to a single topic is all well and good, but as you acquire more document you reach the limit of that shelf. The question can be put off by balancing additional books atop the row of spines, but eventually one must succumb to the inevitable and displace the contents of an adjoining shelf. This is annoying on a personal scale, but could be disastrous on the scale of a library that wishes to maintain proper order. The displaced shelf must displace another, and that a third, until the displacement dominoes through the entire contents of the hall it is in. And this would happen every time a shelf reaches capacity. Well, this is where the minecart tracks come in. Each bookcase will be dedicated to a single category only, and when one reaches the capacity of its case, a new one can be wheeled in. A single moderately-fit librarian with a can of axle grease and the key to release the locking mechanism of the wheels could move entire bookcases around with ease, and with no risk to the secured contents.