Drilling dozens of accurately-placed holes for a shelving project? Try a shelving jig, like this one from Rockler. It’ll keep you from spending your whole day measuring and drilling, and you’re far more likely to end up with level shelves afterwards to boot.
The jig is simple: it’s just a clear plastic plate with pre-spaced 1/4” or 5mm holes that are aligned in two rows. This spacing allows you to drills holes either 1-7/16” or 2-1/2” from the shelves’ front edge. Just hold the bar against the shelving edge, mark, and drill the hole.
And it’s just $25. That sounds like a lot for a little piece of plastic, but this could easily save you hours on a bookcase project. How much is your time worth?