It's like the old carpenter said, as he hammered the screws into place: "Every trade has its tricks but mine."
Here's one. It's not much of a trick, but it helps keep me out of trouble with a router. You usually need to know the distance between the edges of the routed groove and the edge of the baseplate of the router, so you can clamp a guide down at the proper spot. You can calculate it but it's easy to get tripped up -- and why do it on paper when you can create an example and measure it directly? There you go! It's a lot easier to get everything in the right place with an actual example you can plop down on the work and use to mark it. A real pro might even use it to set dividers and eschew the numbers altogether. Me, I'm an amateur and this makes it easier to visualize how things will turn out.
As you might suppose, I'm building more bookshelves. As ever, I find a ratio of at least 4:1 between the measuring/setting-up and doing the cutting.
(The greenish tint to the freshly-exposed wood? It's a scrap piece of poplar, which often looks a bit green. Very nice to work with, harder than most soft woods. Smoothed and linseed-oiled, it will usually retain the hue.)