When assembling a bookshelf cabinet, choose hardware to match the shelves or your décor. If the hardware will be hidden, go for function rather than appearance to ensure that the shelves stay in place.
These are sturdy and permanent. They are visible inside the cabinet and sometimes quite large, but they can be painted or stained to complement any design. Books and other contents of the shelves may hide them as well.
You'll need to drill 4 holes per shelf to use this type of support. This is the type usually found in inexpensive, pressed wood shelf cabinet kits. The wood in these kits has multiple holes so you can adjust the shelves. The chief benefit is that they're inconspicuous.
An adjustable shelving system is handy for any book cabinet. It allows you to move the shelves any time you wish to accommodate items of different sizes. You can even move them closer together and add another shelf. Most adjustable shelving systems do have a visible track, though painting or staining over it can make it a little less noticeable. Books can typically hide the tracks inside a cabinet better than L-brackets.