Design your shelving based on your available space. If you're making built-in bookcases, measure the space available, both at the bottom and the top- don't assume your walls are plumb and square. You will want to break up your shelving with vertical sections every 30" or less- anything more and your shelving will sag. Use no more than 18" to 24" spans for typical 1" wood. If they do start to sag, you can go back and cut small square 1" side panels and mount them vertically in the shelf below for additional support. For built-in bookshelves, design a box to fit the space, and then plan for vertical sections and the shelves to fit into it. You can build it and then slide it into place and mount it.