drill jig to drill shelf holding pins in an adjustable book case. Two on each side, directly from each other. Mine is by Kreg and has two size drill bits for the standard pins most shelves use. Mine can be set for up to 6 holes, drill the first hole, set the pin in it, then drill the next 5, pick up the jig, place the pin in the last hole drilled and do it over and over.
It is easier to use this when building the shelves as there is an attachment to this unit that will let you square it to the front/rear edges of the sides.